Merge Request Queue (See issue #75):

* 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

Code/CoreData/RKManagedObjectLoader.m

@@ -24,7 +24,7 @@
     }
     return self;
 }
-
+    
 - (void)dealloc {
     [_targetObjectID release];
     _targetObjectID = nil;