ISO8601

This addresses #35. Since you [recommended](https://github.com/soffes/ssdatakit/issues/35) using the ISO8601 lib for date parsing it seemed appropriate that SSDataKit use it as well. This PR adds Cocoapods to the test project and then updated the `-parseDate:` method to use the ISO8601 lib for when the input was a string.

~~NOTE: The 2 existing ISO8601 tests now fail, but my added test now passes.~~ Fixed.