add empty, between, and list filters + tests to mongoengine and peewee

This adds the new SQLAlchemy filters in #703 and #711 to Peewee and Mongoengine. It also adds tests for the new filters, adds filtering on timefield to peewee, and removes the datefield converter from mongoengine (mongo doesn't have a datefield).

Here are two examples I used for testing the UI:
* Mongoengine: https://gist.github.com/pawl/dfbc5f3e5e9e093974d3
* Peewee: https://gist.github.com/pawl/ebcd518f03a987d5f7b9