Added macros for dynamic Objective-C properties

Added some macros to simplify declaration of dynamic Objective-C properties using associated objects. Basic syntax is like so:

    // to declare a basic property
    CHPropertyRetainNonatomic(UIView, NSString *, someProperty, setSomeProperty)
    
    // to declare a primitive property (ie. BOOL, int, struct, etc.)
    CHPrimitiveProperty(UITextView, CGRect, pictureFrame, setPictureFrame, CGRectZero)

I find they eliminate a lot of boilerplate code.