Add missing valueWithRange method to NSValue

Method available on both [OS X](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSValue_Class/Reference/Reference.html#//apple_ref/occ/clm/NSValue/valueWithRange:) and [iOS](http://developer.apple.com/library/ios/#documentation/cocoa/reference/foundation/Classes/NSValue_Class/Reference/Reference.html#//apple_ref/occ/clm/NSValue/valueWithRange:)