Mongo-related changes

* Added a RootDocumentModel for MongoKit so that we can easily
implement system-wide defaults for Mongo calls. Every other object
model in the database should inherit from this model.
* Implemented a few functions to do start standardizing some regular
calls.
* Added a save() override function to RootDocumentModel that
automatically sets the last_modified datetime for a Mongo Document so
that we don’t have to think about it.
* Code cleanup.