Introducing $GITHUB_DOMAIN env variable.

- Lets users set a github domain variable to use corporate githubs.
- Defaults to github.com
- Note the git:// change in the bower install.  This was necessary on
  our internal servers. This should probably not be kept in if I want to
do a pull request. see:
http://git-scm.com/book/en/Git-on-the-Server-The-Protocols