Support AppKit

Hi there,

Just want to say I am a huge fan of newsyc for iOS and it's inspired me to write a client for Mac. 

I've done a quick port of HNKit to AppKit. All of the UIKit specific code is included when the IS_MAC_OS_X pre-processor symbol is not set.

Thanks for the code and heres a quick preview of what I'm building!

![Hackernews 2013-02-27 22-24-17](https://f.cloud.github.com/assets/50156/200621/42d4f54a-80d0-11e2-8f08-7c519409e8bd.png)