OS X Support

Added support for using HearthStats.net Uploader on Mac OS X.
Implemented using native calls to the Core Graphics library via JNA and
Rococoa, it provides the same image output as the Windows code so the
rest of the app is able to handle it without further changes. There are
probably some more optimisations possible (such a better way to convert
from OS X’s ARGB to Java RGB images) but it seems fast and reliable
enough to start using now!