Closes #560

This implements removing of the shims path element via bash
substitution, instead of jumping around in all the `$PATH` elements.

Running any shim (and thus `rbenv-exec`) would always execute
`rbenv-version-name` twice: once in `rbenv-exec` and another time in
`rbenv-which`, even though RBENV_VERSION variable would have already
been populated at this point.

Now RBENV_VERSION is respected within `rbenv-which`.

Fixes #626

The previous Makefile only worked on OS X. The dynamically generated
Makefile (from `Makefile.in`) should now work on multiple platforms
(tested on OS X and Ubuntu).

The `shobj-conf` script imported from bash seems to not support the
latest OS X. This makes sure that `SHOBJ_LDFLAG=-dynamiclib` is output
for Darwin10+ (latest version is Darwin 13.0).

Given the `-o <HOST-OS>` parameter, the script generates environment
variables with information how to compile dynamically loadable libraries
for that system.

Imported from bash-3.2.48

Seems to only work on OS X; fails on Travis
https://travis-ci.org/sstephenson/rbenv/jobs/16384866

It's slow and not necessary since we expect `$0` to already be expanded.

In tests this change forces us to deal with some relative paths, but
it's not a big deal. The `rbenv init -` output in the most common case
will be the same as before:

    source '/home/myuser/.rbenv/libexec/../completions/rbenv.bash'

This speeds up every `rbenv` invocation significantly.

With `realpath` extension, hooks tests on OS X will output
`/private/tmp` instead of `/tmp` because the latter is an actual symlink
to the former.

Avoid this mistmach in output assertions by expanding BATS_TMPDIR if
`realpath` extension is compiled.

On systems that support both C compiling and dynamic loading, we can
speed up `realpath()` (where most time in rbenv is spent) by replacing
it with a dynamically loaded bash builtin.

When `make -C src` is called in the project's root,
`libexec/rbenv-realpath.dylib` will be created. If it exists, rbenv will
attempt to load it as a builtin command. If it fails, execution will
fall back to the old `realpath()` shell function.

Do not quote completions in $reply

pyenv-version-file-write: do not use rm, but clobber the file

With e.g. /usr/local/bin/.python-version owned by some user, `pyenv
local foo` would fail, if the user has no permissions for
`/usr/local/bin`, but only the `.python-version` file.

With an empty list of completions, zsh would complete a space / advance
the cursor by a space when completing.

doc: note about using multiple versions

Add a note about how to activate/use multiple versions to the main
README, to the "Choosing the Python Version" section.

This release of Jython requires JDK 7 or above

Setting MACOSX_DEPLOYMENT_TARGET on Darwin