MongoDB Raw Query Parser

Ok, here's the bundle of changes related to the query parser you wanted. Don't have full docs written for them yet, but I'm working on some internally that I can share when they're done. Basically the goal was to provide an alternative to QuerySet that allows expression of any query MongoDB can support and lower-level access to pymongo when it's needed.

Supports validation, db_name transformation, and updating the document in memory (for a few operator types; more could be added).

Getting unit test fails because of documents not being registered (I'm guessing my tests just have to be changed to work with whatever the latest version of MongoEngine is, but if there are more fundamental incompatibilities, let me know and I can try to help debug).