Replace JSONKit with NSJSONSerialization.

This change is prompted by a couple of issues:

* JSONKit is, I believe, no longer under active development.
* As of Xcode 5, JSONKit [is no longer warnings-zero](https://github.com/johnezang/JSONKit/issues/133).

Additionally, targeting 10.7+, `NSJSONSerialization` is available to use instead.

To minimise the impact of the change, I made the changes as categories on `NSString`, `NSDictionary` and `NSData` to match the JSONKit API.