Adding the ability to install plugins from git.

Thanks to the switch to bundler, this should expose the ability to install vagrant plugins from git sources (including github). If you install a plugin with the ```--plugin-git``` option it will use that as a git source in the resulting bundler file. If you want to specify a ref, tag, or branch, you can do that with ```--plugin-git-ref```, ```--plugin-git-tag```, or ```--plugin-git-branch``` respectively.

This should be a huge benefit to anyone running internal plugins, where having a private github repo is much easier than having a private gemsource.