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/ObjectMapping/RKObjectMapper.m

@@ -139,7 +139,6 @@
         [self addErrorWithCode:RKObjectMapperErrorObjectMappingTypeMismatch message:errorMessage keyPath:keyPath userInfo:nil];
         return nil;
     }
-    
     for (id mappableObject in mappableObjects) {
         id destinationObject = [self objectWithMapping:mapping andData:mappableObject];
         BOOL success = [self mapFromObject:mappableObject toObject:destinationObject atKeyPath:keyPath usingMapping:mapping];