0.5.0: Aggregation

• Added new methods Collection::distinct() and Collection::count()
  (@thekid)
• Added new method Collection::aggregate() to perform aggregations, see
  https://docs.mongodb.com/manual/reference/command/aggregate/ and
  https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/
  (@thekid)
• Fixed all places where _id was assumed to always hold ObjectIds.
  (@thekid)