Renamed the repo to omniauth-radius-rails

For a couple of reasons:

1. Lets us have this engine public to make it easy to install on the
server. Nothing in here is sensitive, just oauth boiler plate.
1. It is editable by anyone in our github profile. So we don't have to
share rubygems.org credentals, or add people to the project.
1. The new name shouldn't raise questions. It fits in with our omniauth
provider repo.

Oh, added a few specs as well.
Adding test coverage to ported code
Renaming gemspec to omniauth-radius-rails
Adding test coverage for extracted code