Add initial support for Groups

[nixpanic]

This adds support for the following functions that act on a Group:

• Bridge.GetAllGroups() Returns an array of all the groups that are known to the bridge (this function has been renamed from Bridge.GetGroups() to match the Bridge.GetAllLights(), GetAllScenes() naming scheme)

• Bridge.GetGroupByName(name) Returns the group with the given name, or an error if not found

• Bridge.NewGroup(name, class, lights[]) Create a new group

• Group.Delete() Remove the group from the bridge

• Group.On() Turn all the lights in the group on

• Group.Off() Turn all the lights in the group off