Decouples error handling
Adds new NSError category
Adds Description to SPChangeProcessor errors
Minor enhancements
Updates SPChangeProcessor API
Implements Processor's HALT Mechanism
Merge remote-tracking branch 'origin/issues/254-data-duplication' into issues/208-performance

Conflicts:
	Simperium/SPChangeProcessor.h
	Simperium/SPChangeProcessor.m
	Simperium/SPWebSocketChannel.m