Replace JSONKit with NSJSONSerialization.

[marramgrass]

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. Obviously, this is saddening - especially when treating warnings as errors ;)

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.

[nfarina]

This is great, thanks so much!