Enhancements and fixes to MongoDB support

Hey there,

we required various enhancements to the fine `Robotframework-Database-Library`:
- Keyword `Retrieve All MongoDB Records` and `Retrieve Some MongoDB Records` are now able to return records as a list of native PyMongo documents instead of just concatenating them into a string; the API is backward-compatible by having a new kwarg `returnDocuments=False`
- Fix: ObjectId now lives in `bson.objectid` instead of `pymongo.objectid` for recent versions of PyMongo
- Fix: setup.py should not import `DatabaseLibrary` for retrieving the modules' version

Cheers,
Andreas.