Remove WaitOp and WaitAllOps, preparing to return Futures everywhere and deprecate Op
Tiny fetch_next docs improvement
Return Futures from all public Motor async methods

Sorry for the huge diff:

* required callbacks are now optional
* if no callback, async methods now return Future
* GridFSHandler.get_gridfs_file returns Future, accepts no callback
* del most of Motor's test framework in favor of Tornado 3's
* to_length() doesn't just warn you if there's no limit, limit is required