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.
Adding pylintrc config and custom runner and making it pass.

Some key things to notice:
- The tools (pep8 / pylint) disagree about continutation lines.
  This is because PEP8 is ambiguous about "multi-line constructs".
  One of the examples for bracket indentation in the PEP is

      my_list = [
          1, 2, 3,
          4, 5, 6,
          ]

  but the PEP doesn't explicity say whether or not

      my_list = [1, 2, 3,
                 4, 5, 6,
                 ]

  would be allowed. As it turns out, pep8 (the tool) considers
  the latter to be a continuation while pylint does not.
- I changed all indentation where this was a problem, but was
  intentionally inconsistent. This way we can see "all options"
  and discuss whichever is preferred.
- I made a _MetadataMixin helper class for methods shared between
  storage.Bucket and storage.Key. I did not refactor the affected
  tests but they do still give 100% coverage.
- To silence docstrings warnings I added many bogus docstrings
  without actually documenting the given objects.
- I factored out part of
      storage.connection.Connection.generate_signed_url
  into a _get_expiration_seconds() method and added tests.
  This method is untested (has pragma no cover) and is still very long.
  I refactored because pylint complained about too many locals.
- I made sure tox -e lint and tox -e cover passed.
- I increased the maximum number of arguments from 5 to 10 and
  the maximum public methods from 20 to 30. Offenders were:
    - storage.connection.Connection.make_request (6 arguments)
    - storage.connection.Connection.api_request (9 arguments)
    - storage.connection.Connection.generate_signed_url (6 arguments)
    - storage.bucket.Bucket (27 public methods) and
    - storage.key.Key (23 public methods)
  I think refactoring these should be considered (though is not
  mandatory). I certainly don't know a good solution for the class
  public methods.