Merge pull request #83 from dhermes/refactor-env-name

Refactoring _get_environment.
Moving GoogleCredentials.get_application_default into helper.
Refactoring _get_implicit_credentials into component pieces.
Merge pull request #84 from dhermes/refactor-get-app-default

Refactor GoogleCredentials.get_application_default
Supporting client secrets / user flow with a hack.
Adding a script and example creds file to check user implicit auth.

Also fixing bug where SETTINGS was immutable.