add `global-rbenv-mode` to toggle rbenv.el
Merge pull request #1 from senny/rbenv_minor_mode

rbenv-minor-mode to display the active ruby version in the modeline
allow multiple locations for the global rbenv version file.

closes #2

By default rbenv.el will check for the global version file in the following locations:
* ~/.rbenv/version
* /usr/local/rbenv

You can modify the list `rbenv-global-version-files` to change the locations where rbenv.el
will look for your global version file.
Merge pull request #3 from senny/multiple_global_version_locations

allow multiple locations for the global rbenv version file.
Revert "Merge pull request #3 from senny/multiple_global_version_locations"

This reverts commit f779f8bb7e4159dc5cce0b3158b11e993e43f62e, reversing
changes made to 763ca00aa534ae24c53db170aca812c078877682.

The details in #2 were insufficient and led to a useless feature.
We don't only need to modify the location of the global version file
but more where the complete rbenv installation is located.
make the rbenv installation directory configurable.

closes #2