Update code to support HTTP caching.

Added GTWHTTPCachedResponse class with 304 status code.
Added GTWHTTPDataResponse class with lastModified ivar.
Added lastModifiedDateForQuadsMatchingSubject:predicate:object:graph:error: method to SPKQuadModel.
Added stub lastModifiedDateForQuadsMatchingSubject:predicate:object:graph:error: method to SPKTripleModel.
Added accessPatterns SPKTree category method to NSObject.