Removing unused variables detected by pyflakes.
Fix lint failure due to long line.
Merge pull request #222 from dhermes/fix-pyflakes-errors

Removing unused variables detected by pyflakes.
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.