Adds ability to do a blind update rather than insert for a model

Based on [this thread](https://groups.google.com/forum/#!topic/cqlengine-users/emMyqOZn2A8) from the google group.

This is a first pass to get feedback, I can go back in and add tests and such if it gets accepted.  It's already fully working as far as I can tell - calling save with `update_only=True` does an update rather than insert.