support for rbenv

Since I could not set the RBENV_VERSION, even with the command rbenv
shell, I just used the PATH for the specific ruby binary. It works well
for me.

Also I hope I haven't break the rvm :wink2:
updated README
updated gem description
choosing the most recent minor version for a given version when using with rbenv
Better way to get rbenv installed versions and fix bug introduced

Get versions: Now it will get rbenv versions when started instead of
getting inside the threads.

Fix: Prepended each rbenv version name with a "_" so we don't use
rbx-2.0.0 instead of 2.0.0 for example.
Updated README
Use regex instead
Split stuff into methods
moved jruby to be the last to be executed
use ruby bin on shims instead. Thanks to @mikej
Fixed clash
Prepare for 0.0.4 release, pending more testing