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
Fixing code that causes test_oauth2client to fail.

In particular:
- Removing executable bit from "test_oauth2client".
- Temporarily disabling slow (network bound) tests.
- Making client.SETTINGS a class so it can be mutable.
- Making GoogleCredentials._get_implicit_credentials a classmethod
  so it can access `cls`.
- Changing use of `urlparse.parse_qs` to `urllib.parse.parse_qs` (did
  not get converted during Py3K switch).

Partially addresses #85. Not closing out until slow tests are
re-enabled and other test failures of `py33openssl14` and
`py34openssl14` pass.
Temporarily disabling py33openssl14 and py34openssl14 environments.
Merge pull request #86 from dhermes/fix-immutable-settings

Fixing code that causes test_oauth2client to fail.