Replace CJSONSerializer and CJSONDeserializer with native NSJSONSerialization

Hey guys,

This is a pretty quick and simple replacement. It also eliminates 10 files completely. I've tested it in a small app to make sure that events go through properly to your hosted service... no compilation errors, etc.

Another thing I noticed while looking this over is that I think you'll get crashes if/when people try to send NSDate or NSURL objects as custom event properties or global user properties (because they don't have default JSON serializations). Mixpanel is good about checking for these object types: https://github.com/mixpanel/mixpanel-iphone/blob/master/Mixpanel/Mixpanel.m#L209 Seems like a wise thing to do.

Hope the code-base reduction is helpful :)

Cheers,
Peter