Filter parameters config

This pull is related to https://github.com/doctrine/mongodb-odm/issues/899
I'd much rather see this as an example in the documentation, akin to what exists for ORM: http://doctrine-orm.readthedocs.org/en/latest/reference/filters.html

It's trivial for users to create model-specific filter classes. Also, I imagine that the simplest BsonFilter would take a single array/object parameter. `addFilterCriteria()` would then do an `instanceof` check and conditionally return the raw criteria parameter.