Refactoring _get_environment.

- Removes use of "global" by putting the env name in another
  namespace.
- Splits logic into digestible pieces.
- Also makes slight change to GoogleCredentials docstring.
Moving GoogleCredentials.get_application_default into helper.
Refactoring _get_implicit_credentials into component pieces.
Supporting client secrets / user flow with a hack.
Adding a script and example creds file to check user impliccit auth.