added support for layer groups

Not sure if this is the direction you would like to go with this.  My particular problem is that none of the cartodb layer types fit my work flow.  We use cartodb to produce layers at runtime, using the [Core API] (http://docs.cartodb.com/cartodb-platform/cartodb-js.html#core-api-functionallity).  Using this technique, we do get Urls for the tiles and utf grids.  But then I end up with separate items in the layer selector, which is not attractive.

As best I can tell, there is not currently any way to leverage the 'group' layer type.  So hopefully this helps.