Adding omniauth support for Google Apps authentication (Issue #1635)

Adding support for omniauth Google Apps strategy so that people can restrict the login to their own Google Apps domain.

The changes in config/initializers/devise.rb must seem like a hack, but I an pretty new to gitlab's code and couldn't possibly figure out how to put that google_apps support in so had to make that hack. If someone can revise that and put the right logic, it would be great.

Thanks !