Switching Cordova-used JSONKit methods to use NSJSONSerialization

NB: I'm only overriding the behavior for the JSONKit methods that Cordova uses.
@sholly-sfdc I think it's fairly safe in this direction (Cocoa objects -> JSON), as errors are either going to be unrelated to specific data (memory overflows and such), or are going to fail fast on invalid data types being attempted to be converted into JSON.  But I suppose I could leave the error messages out...they're probably not going to give a lot of useful information anyway, in the cases where the serialization fails.