50%: Multiple datasources

This PR replaces #27: it's more of a drop-in replacement for the current MongoTripod (although the configuration and signature for most delegates has changed).  

There are some semantic changes to Tripod concepts: instead of "databases" it internally refers to them as "groups" and it replaces "collections" with "pods".  This is an effort to detach the Tripod concepts from any particular database concept.

Some unit tests are still needed, but I think this is ready for some vetting.
I would prefer ``$groupName`` to ``$group``, as the latter is definitely going to get confused with grouping in dbs, although I agree with @rgubby that it would be good to come up with a new name entirely is possible.