Explicitly #import <Cocoa/Cocoa.h> instead of assuming it has already been imported

I tried using the library in an OS X project that [does not have a precompiled header](http://qualitycoding.org/precompiled-headers/), but compilation failed. `TARGET_OS_IPHONE` and `TARGET_OS_MAC` were unresolved because I do not have the Cocoa headers implicitly imported in every file.