Added shouldRespectCacheControlHeaders property and PrivateMethods category for subclasses.
Added some debugging info and made the canonicalRequestForRequest: method to be called in subclasses.
Merge pull request #21 from arturgrigor/master

Allow SDURLCache to be subclassed instead of hijacking SDURLCache's methods
Conditionally release GCD object. Fixes #22
pull from the latest steipete/SDURLCache and fix conflicts