migrate to Autorelease Pool Blocks

Xcode  | OS X | iOS
------------- | ------------- | ------------- 
Xcode 4.2+  | MRR: all releases, ARC: 10.6+ | MRR: all releases, ARC: iOS 4+

[See the Objective-C Feature Availability Index](https://developer.apple.com/library/ios/releasenotes/ObjectiveC/ObjCAvailabilityIndex/index.html) and [Using Autorelease Pool Blocks](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAutoreleasePools.html) for more detail.