Changing auth to use implicit environment.

FWIW the only real code changes occur in

- `gcloud/credentials.py`
- `gcloud/datastore/__init__.py`
- `gcloud/datastore/demo/__init__.py`
- `gcloud/storage/__init__.py`
- `gcloud/storage/connection.py` -- Some helpers added here should probably be moved elsewhere. Maybe I file a bug?
- `gcloud/storage/demo/__init__.py`
- `regression/regression_utils.py`

Also note that google/oauth2client#93 may be problematic for us, but only in the short term (on the order of days).

Also, I still need to update the encrypted environment variables on Travis.
Hmm, if we monkey-patch `_get_signed_query_params`, we should be able to drop the others (they are only used by it or `_get_pem_key`, which only it calls).