Removing gcloud.credential.Credential class.

No instances of this class were ever created and it only had
one factory method, which is now a function. This class was
also misunderstood / misreferenced in the docstring mentions.
Updating docstring on credentials.get_for_service_account function.
Merge pull request #235 from dhermes/remove-credential-class

Removing gcloud.credential.Credential class.