Closes gh-47. Initial implementation of OS X build integrating changes submitted by Felix Holmgren (https://github.com/Felixyz/RestKit).

* 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.

Code/CoreData/RKManagedObjectStore.m

@@ -7,7 +7,7 @@
 //
 
 #import "RKManagedObjectStore.h"
-#import <UIKit/UIKit.h>
+#import "RKAlert.h"
 
 NSString* const RKManagedObjectStoreDidFailSaveNotification = @"RKManagedObjectStoreDidFailSaveNotification";
 static NSString* const kRKManagedObjectContextKey = @"RKManagedObjectContext";