* 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
* Factored out display of alerts into RKAlert interface that hides the differences between UIKit and OS X Cocoa.
* Added macosx to supported platforms to enable build on OS X.
* Configured project to use conditional architectures to enable building on OS X and iOS from the same targets.
* Implemented a bare-bones OS X example app.
* Create `rake build` task for building RestKit against iOS and OS X SDK for quick testing.
Introduces four modes for handling background requests:
* RKRequestBackgroundPolicyNone - The default behavior replicating pre-background behavior. No special action is taken with regards to backgrounding.
* RKRequestBackgroundPolicyCancel - On transition to the background, requests with this policy set will be cancelled automatically and the delegate informed.
* RKRequestBackgroundPolicyContinue - Requests with this policy will be continued in the background after the app has been transitioned.
* RKRequestBackgroundPolicyRequeue - Requests with this policy will be cancelled and then immediately placed onto the queue for processing the next time the app is returned to the foreground.
