Ability to get array of Group names

I needed to create this functionality for myself so just putting it up here.

I often need to check if a user is in multiple groups which I need a flat array of names to easily compare with.

No experience with Kohana but I added the method there as it should be fine given its just using existing method calls to get data and returning an array.

Havnt added tests because I have no mocking idea whats going on in there :D