Removed dependancy on custom MonoMac build

Looks like converting it to unix line ending has caused some oddity with the diff :-1: 

I've changed added a static class 'NSGraphicsExtensions' which contains your previous MonoMac NSGraphics changes.  

This seems to resolve the problem and means you can use the library with Xamarin.Mac as well! I think it might need some testing though as I only confirmed it built and looked alright.