Added OS X support

I've added the some modifications to add OS X compatibility.

In my OS X projects I have used a modified version of UIView-Autolayout for some time now and it has worked very well.

I believe that other OS X developers would also benefit if the changes become public and official.

Besides changes in the code, it may needs to include examples for OS X and maybe even unlink the name from iOS ("UIView") and make it more generic.