Switched some NSAutoreleasePool calls to @autoreleasepool 

Increased efficiency as per https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSAutoreleasePool_Class/Reference/Reference.html#//apple_ref/doc/uid/20000051-1722

I realise I have made a change to JSONKit but as the original code hasn't been modified in a couple of years I figured this was fairly safe...