Adding support for limits and sorting

[vzsg]

This PR resolves #24 by handling the Query limit and sorts fields inside MongoDriver.

For select queries:

• The Query.sorts array is used to build a sort document in the following format: { "field1": 1, "field2": -1, ... } (depending on sort order), and passed to MongoKitten's find method.
• If Query.limit is set, the count and offset parameters are passed directly to MK find.

For delete commands:

• If no filters are set and limit is 0 or not set, the whole collection is dropped (previous behavior).
• If there are filters and the limit is 0 or not set, every matching document is removed (again, previous behavior)
• If the limit is 1, a single matching document is removed.
• Every other limit value is considered invalid and an error is thrown before calling MK. (This is to comply with the delete command's specification, which only allows these two values – and would fail on the driver level anyway.)

[tanner0101]

Awesome, thanks guys!

[tanner0101]

https://github.com/vapor/mongo-driver/releases/tag/1.0.8