`rbenv version` shows symlinks acting like aliases

Show symlinks acting like aliases under `$RBENV_ROOT/versions`:

![screen shot 2014-10-16 at 12 16 41 pm](https://cloud.githubusercontent.com/assets/887/4661143/03af6870-5520-11e4-87a5-2e368cb5e1ee.png)

Before this symlink aliases would display just like any other version, making it hard to distinguish between "real" versions and symlinks that are just shortcuts.

:construction: Proof of concept. Needs code cleanup, tests. :construction: 

This is slated for a post-1.0 release. In discussion with @sstephenson we thought that it would be a good idea to pair this with an `rbenv alias` command to manage such aliases.