Added features, refactored gem, and ported it to Bundler

@rdp,

Thanks for your gem!! I'm making good use of it in my daily Ruby activities.

I took it on myself to dust off this gem and add a few things that might be useful to others:

`-b --branch` option as requested here: #5 (Though I added feature and then saw #5, so I can promise 3+ people want this feature )

`-l --location` option - added Github shorthand format, i.e. `rdp/specific_install`

Added an alias for `gem specific_install` which is `gem git_install`. (#1)

Some refactoring... and swapped it over to Bundler conventions since Jeweler has fallen out of favor in Ruby world.

Oh yeah, also added a bump to 0.2.4 since this seems like a significant set of changes... maybe you'll even dare to bring it to 0.3.0 at your discretion.

I'll send a second pull request for README revisions to reflect the changes.

-ZPH