* Introduces RKRequestCache for cacheing responses (supports ETag conditional GET, use cache if available, use cache on error, etc.) closes#75
* Updates to Three20 layer to eliminate need for intermediary TTTableItem classes closes#76
* Fixes to ensure iOS 3.x compatability:
* Switched compiler to Clang
* Updated conditional checks for UIBackgroundTask symbols to ensure runtime safety on iOS 3.x
* Removed unnecessary linkage against UIKit and CoreFoundation from library targets
* Fix for issue where RKRequest objects could become stuck in infinite loop within RKRequestQueue loadNextInQueue if you start
a request and then cancel immediately. On cancel only decrement loadCount if the request has start loading. refs #122
