add uniqueness constraint to provider and username
Simplify OmniAuth auth stubbing

There is now a single file that is responsible for defining and enabling
OmniAuth stubs. Furthermore, authentication in feature specs is more
explicit with regard to which user is signed in, and configures OmniAuth
so that the about-to-be-signed-in user can sign in via Chef and can link
her GitHub account

Conflicts:
	spec/spec_helper.rb
Merge pull request #329 from opscode/unique-account-provider

Make the provider/username combo unique