Fixes ARC and project compatibility

1. Fixed CHMutableDictionary, CHBinaryTreeNode, CHSinglyLinkedListNode, and CHDoublyLinkedListNode object to be __unsafe_unretained for use with ARC
1. Removed hidden visibility for kCHPointerSize as the symbol was being duplicated in dependent projects
1. Added CocoaPods CHDataStructures podspec
1. Updated CHDataStructures to be 10.8 compatible
1. Fixed framework search paths for CHDataStructures-iOS project