First pass at adding group_id

In order to better support the dichotomy of org_id:app_id paired information within the Geotrigger Service, we wanted to add an additional axis of data separation into rainman. In order to make this more generally applicable, we decided to add the concept of a group_id to rainman. The group_id should be able to be left blank and have everything work as necessary, or it can be used to further segment data.

Haven't tested this at all yet -- wanted to get some feedback.