Add auto switch script

Switch gem environments automatically with the contents of a .ruby-gemset file.

    $ ruby -v
    ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-darwin13.0]
    $ cd old_rails_app/
    $ ruby -v
    ruby 1.9.3p545 (2014-02-24 revision 45159) [x86_64-darwin13.0.0]
    $ echo $OMG_NAME
    rails32

Requires that the `ohmygems_auto` file be sourced in the user's shell.