Add an elasticsearch query builder

Noah is using this for a feature he's working on and it seems relatively stable now, so I figure it's a good time to merge it in.  Happy to get feedback on it and iterate.  Includes tests.

The es_query file includes some documentation on usage, and you can see it in action by looking at tests.py.
specifically should be something like https://github.com/dimagi/commcare-hq/blob/50aa2e53e2403fb33d8d9cfac67a5dcd84a843ea/corehq/elastic.py#L196-196