fix Zeroing Weak References problem

In Apple's [Transitioning to ARC Release Notes](https://developer.apple.com/library/ios/releasenotes/objectivec/rn-transitioningtoarc/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011226-CH1-SW11):
> Note: In addition, in OS X v10.7, you cannot create weak references to instances of NSFontManager, NSFontPanel, NSImage, NSTableCellView, NSViewController, NSWindow, and NSWindowController. In addition, in OS X v10.7 no classes in the AV Foundation framework support weak references.

So when I use Masonry on a NSTableCellView, my program will crash exactly like [this problem](http://stackoverflow.com/questions/10904990/xmpp-objective-c-library-cannot-form-weak-reference-to-instance-0x7fcd4a49893) .

Please see if that is this the right way(`just change weak to unsafe_unretained`) to solved the problem. Or still use weak references for 10.8+ since these are supported and are safer and use unsafe_unretained for 10.7 .