Added LayerGroup and tests

Related with issue #22.

Thanks to your awesome class strutures, adding layerGroup support was this easy.
This is really awesome.

Since `GroupLayer` is so minimal, its tests are also minimal. I'm basically testing the `_newLayer` method.
The logic is almost entirely on Collection, Container and Layer classes.

I leave up to you the naming and if this should be a class or mixin, but I think that using a class reflects closer the leaflet structure.

Many thanks.