Add support for sorting

Allows for single and multiple sort specifiers of types:
- Property name
- Property name and direction (dictionary)
- NSSortDescriptor

Did not implement any parsed string formats for the sake of simplicity
but they could certainly be implemented later without breaking
compatibility.

Added ordering method variants where they seemed useful while
preserving all existing method signatures for backward compatibility.
Implemented code review comments
Improved ordering tests
Merge pull request #57 from jeremymailen/feature/order

Add support for sorting