Adding PEP8 only lint rule to tox and Travis.
Adding verbose flag and fixing PEP8 violation in test.
Adding *egg files to pep8/lint blacklist.
Merge pull request #217 from dhermes/add-tox-lint

Adding PEP8 only lint rule to tox and Travis.
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.