Documenting the skip-existing option for install

Updated Mac OSX brew install command

It seems rbenv now comes with ruby-build.  I have not investigated fully, but the previously shown command: `brew install rbenv ruby-build` caused issues on my machine.  After uninstalling both and simply running `brew install rbenv` everything worked fine.

```
/home/yyuu/.pyenv/versions/3.2.6/lib/python3.2/site-packages/pkg_resources/__init__.py:85: UserWarning: Support for Python 3.0-3.2 has been dropped. Future versions will fail here.
  warnings.warn(msg)
```

`rbenv shell -` allows you to switch to the previously activated ruby
version.  Similar to `cd -` or `git checkout -`.

This tries to implement `rbenv shell -` as proposed in #854.  However,
adding support seemed to break the "shell change version" test.  I'm not
very good at Bash programming, can someone tell me what is wrong with
what I'm doing?  I'd like to add a bit more functionality to this, but
I'm really just cargo cult programming Bash.

Thank you!

fix tests

Extract hook helper

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

Spaces in hook path is tested in test/hooks.bats

Remove support for legacy global version files

Remove all handling/support of .rbenv-version files

`default` was made legacy back in 2011 with
5be66da9f4 (the command was renamed from
`rbenv-default` to `rbenv-global`, and so the global file was renamed
from `$RBENV_ROOT/default` to `$RBENV_ROOT/global` (the latter taking
precedence)

`global` was then made legacy about a month later in Sep 2011 when the
preferred filename was changed to `$RBENV_ROOT/version`.

It was useless for a while now. I should have nuked it prior to the
1.0.0 release but I forgot :(

[ci skip]

Closes #532

This compiles the `realpath` dynamic extension for bash which speeds up
symlink resolution. If the extension doesn't compile due to
cross-platform issues, rbenv will still work normally, although not as fast.

Closes #730

Handles situation when the output is `/bin/shell- args...`. First strip
away the arguments, then the trailing dash.

When invoked from a shell script, `$(rbenv init -)` did not get the
shell name correct.
It needs to look at the `args` value from `ps`.

Ref: https://github.com/yyuu/pyenv/issues/373

References #730

The `../libexec` dance isn't necessary here. It was only necessary in
main `rbenv` command because that one might have been pointed to
directly via a symlink.

It doesn't try to chdir into RBENV_ROOT anymore because that might be
a location of an unrelated rbenv install that might have a different
version than the current one that is installed e.g. via a package
manager such as Homebrew.

Now just tries the repo where the source files (`libexec/*`) are
located, and if that isn't a valid rbenv repo, bail out early.

Closes #739

No need for helper function that's gonna be used just once.

It's only supposed to be set from `version-origin` hooks, but not
inherited from environment in case it was set.

Expose a `version-origin` hook.

It is invoked *before* the traditional `rbenv-version-file` lookup. Because `version-origin` is traditionally run immediately after `version-name`, then any plugin hooks that alter `version-name` would have done so. Thus, running `version-origin` prior to printing the origin gives those plugins a chance to alter the `version-origin` to match.

If any of the hooks set `$RBENV_VERSION_ORIGIN`, then it is used as the return value. Otherwise, the existing logic continues to return "environment variable" or "filename" as appropriate.

This change, in conjunction with the `version-name` hook, makes a clean seam by which plugins can inject their own ruby version setting logic. Using this seam, as opposed to altering `$RBENV_COMMAND_PATH` from the `which` hook, means that the version name and origin are set more reliably and so `version`, `version-name`, `version-origin` and `which` all work as expected. Indeed, even PS1 works now.

Expose a `version-name` hook.

It is invoked *after* the traditional `RBENV_VERSION` lookup. Which means hook scripts can interrogate `$RBENV_VERSION_FILE` and/or `$RBENV_VERSION` (or use the executables).

The hooks are then run, giving plugins a chance to alter `RBENV_VERSION`. Once the hooks have run, we now have (in `$RBENV_VERSION`) the actual version we want to use (or it's empty which defaults to `system` per normal). Lastly, the same logic remains for checking if the version exists, or trimming the `ruby-` prefix.

Prime example: the ruby-bundler-ruby-version plugin can select a ruby by using the `ruby` directive from the `Gemfile` if a local `.ruby-version` doesn't exist.

Fixes #807

Can be used for `.ruby-version` file lookup in the ancestry of a
specific directory. In this mode of operation, global version files
aren't taken into consideration, and the command fails unless a local
version file was found.