LIBCLOUD-625: Allow for internal GCE authorization with metadata service

This PR provides support for authorizing requests from within GCE directly by pulling an authorization token out of GCE's internal metadata services.  This greatly simplifies using libcloud inside GCE since it does not require the user to copy around private auth files.  The only parameter required for this method is the user's Project ID.  See the included docs for a sample use-case.

This PR also attempts to address https://issues.apache.org/jira/browse/LIBCLOUD-607 by appending the Project ID to the cached credential file.
Works for me. The only issue I see is that Google's metadata service requires a custom request header.  I'll make a small change to `get_response_object` to support custom headers.