OS X Support

I've 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!

### Building for OS X

I've updated the build.xml Ant script to build an OS X-specific bundle. I'm not sure whether you can use Windows to build the OS X bundle, I've only tested on a Mac. All you need to do is run the 'build_osx' target in build.xml using Ant. It will output an app bundle in /build/HearthStats.app