Have `rbenv prefix` handle the case where system Ruby is not installed,
i.e. `rbenv which ruby` doesn't find ruby in PATH.

Fixes #362

Provide an `rbenv install` example and explanation lower down, so that
both folk that are following git install and Homebrew install will
finish at the same section and learn how to use ruby-build.

Also remove step 6 of manual install process: `rbenv rehash`. It's
unnecessary at that point, even if they already did `rbenv install`
(which rehashes automatically).

Fixes #455, closes #456

It can be confusing if people mistake the example as instructions to
install ruby-build with.

Don't suggest that they add to their `.profile` anymore because:

1. They would have to restart the desktop session for their change to be
   reflected, or source the file manually;
2. An error in `.profile` may prevent logging in;
3. The `rbenv()` shell function and shell completions won't be available
   in Terminal since it doesn't start bash in login mode by default.

Therefore, suggest that they use `.bashrc` instead. This will be
immediately reflected in a new Terminal tab. If bash is started in login
mode somehow, the default `.profile` is set up to source `.bashrc` anyway.

Also, don't suggest restarting the shell environment with
`exec $SHELL -l`, since we don't know what was the original mode that
their shell was started in. (OS X Terminal.app will be login mode,
Ubuntu Terminal has non-login mode by default.) Mode can be checked with:

* bash: `shopt -q login_shell`
* zsh: `[[ $options[login] = "on" ]]`

But since this is gnarly, let's just avoid it altogether and go the
easy route.

Closes #305, fixes #373, reverts #286

If `rbenv init -` outputs `.` in place of `source` and that gets eval'd
by the desktop manager via `~/.profile`, it chokes and prevents the user
from logging in.

Fixes #457

ksh syntax becomes:

    function rbenv {
      typeset command

`typeset` only declares a local variable if there's an explicit
`function` declaration; otherwise the variable leaks.

Other shells use this syntax:

    rbenv() {
      local command

This is for dash compatibility, which supports neither `function` nor
`typeset`.

references #205, fixes #408

Use process subtitution syntax:

    . (rbenv init -|psub)

instead of:

    eval (rbenv init -)

because the latter doesn't work well with newlines.

fixes #450

fixes #444

Closes #432 [ci skip]

Homebrew homepage moved to a new location

readlink comes from GNU coreutils.  On systems without it, rbenv used to
spin out of control when it didn't have readlink or greadlink available
because it would re-exec the frontend script over and over instead of the
worker script in libexec.

Fixes #389

Making empty commits in tests blocked by opening the default text
editor, waiting for a message which we specified to be empty.

http://thread.gmane.org/gmane.comp.version-control.git/225529

closes #379

It was a dumb idea and it wasn't even implemented perfectly.

Per https://github.com/ruby/ruby/blob/7d3db3c/ruby.c#L1383-1391

Enable JavasCript, Lua and Erlang scripts to provide completions

Fixes #344, #196

replace tab with spaces

Fixes #238