Cocoa window support

So it contains basic cocoa window support, and code for event handling, though it needs to be filled.
I've only done left/right mouse click support, as it seems that there's some change in how the input is gonna be handled, so I'll implement the new version directly.
Also I've pushed on master instead of cocoa, because the OSX only crates won't tried to be compiled on other platforms (I've tried on linux and it works fine).