DATAMONGO-954 - Add support for system variables in aggregation operations.

Add assumes for appropriate MongoDB version.
DATAMONGO-970 - MongoTemplate.remove(…) now correctly builds query for DBObjects.

If a DBObject was handed into MongoTemplate.remove(…) we previously failed to look up the id value to create a by-id-query. This commit adds explicit handling of DBObjects by looking up their _id field to obtain the id value.