Cache element identification & targeted invalidation.

Changes:
RFWebServiceCache cacheIdentifier attribute added.
RFWebResponse dynamic cacheIdentifier property added.
RFWebServiceCachingModel extended with an additional field called cacheIdentifier. 
RFWebServiceCacheContext migration handling added to the persistent store.
RFWebServiceCachingManaging protocol extended with set, get, flush and identifier parsing methods.
RFDownloader modified to set cache data with identifier. To this the method parameter values are set as a values property to the RFDownloader instance.