Add support for Mongo-Thingy

[ramnes]

Hey there, thanks for the great library!

Mongo-Thingy is a sync + async ODM that already has Mongomock and Motor as supported backends, so I really would love to add mongomock-motor to the list. :)

This PR fixes the errors that are raised by our test suite when adding mongomock-motor to the list of tested backends (see here if you're curious).

Basically, we just handle a few more things that should be interesting in other situations too:

• AsyncCursor.clone() and AsyncCursor.distinct() that are missing;
• AsyncMongoMockCollection.database that doesn't return an AsyncMongoMockDatabase but the underlying class instead;
• as well as AsyncMongoMockCollection.__eq__ and AsyncMongoMockDatabase.__eq__ that are missing.

It also adds a test file for Mongo-Thingy, inspired by test_umongo.py, in case you want to keep the compatibility alive.

[michaelkryukov]

Hey, thanks for kind words! Changes look good. I'l release a new version as soon i'l have an opportunity