Refactored group support into it's own Usergrid object.

Like we talked about last week I refactored group support off of the Usergrid.Client object and onto it's own Usergrid.Group object. Few things to note.

* It inherits from Usergrid.Entity. We get a lot of the good stuff from the for the group object itself. We can set, save, and all that good stuff.
* We override fetch. Fetch retrieves the group object, and all members of the group
* add and remove are the way to manipulate group membership easily.