Initial commit of support for Google Compute Engine.

The unit tests are still in development and will be coming in a separate commit.

This currently works, and does not rely on any additional third-party packages, with one exception:  If you want to use the "Service Account" OAUTH authentication, you will need the PyCrypto package installed.  However, it fails gracefully if it's not there.

I'm particularly interested to see if you notice any places where I broke the established API.  I believe that I followed it as closely as possible, however.