Move fatal Pusher errors into a separate domain.

This enables clients to check if the error they have received will
prevent reconnecting without having to know the specific range of
error codes.
Remove support for JSONKit.

I've decided to leave the PTJSON abstraction in place for now - it's
doing no harm.
Fixed some 64-bit warnings.
Updated OSX project settings.
Convert to modern Objective-C syntax.
General style updates:
* Synthesize by default
* Underscore prefix private variables
* Move instance variable declarations out of headers and into implementation
* Prefer properties in most cases over instance variables
* Hide interfaces that do not need to be exposed publicly
* Remove conditional compiler checks for pre iOS5