GCE Driver.

This is a basic GCE hosts driver, along with an integration test.

This has been moved over from https://github.com/bfirsh/docker/pull/22

Auth is handled through oauth. A browser window will open when creating a host that negotiates the oauth dance, and the token is cached. The client_id and client_secret currently belong to a test project I created, but they can and should be changed to something more people can access.

You can run the integration test by passing a --project flag to 'go test', and going through the oauth dance. It takes around 5 minutes to complete.

I'm very new to golang, so please let me know if I did anything incorrectly/unoptimally.

I added a new dependency on code.google.com/p/google-api-go-client/compute/v1, but I haven't commited that yet.

Signed-off-by: Dan Lorenc <lorenc.d@gmail.com>