Add support for groups

Hey Sam, I really like what you've done with this gem and am looking at using it for my [SiriProxy plugin](https://github.com/interstateone/siriproxy-hue). I added group support for group 0 (all lights) that includes what's available with v1.0 of the API. I'd understand if you wanted to wait until the full group API was released or until a test framework was setup. Let me know if there's any changes I should make!