Update, including OS X and Linux support

We recently finished porting NightSky to our cross-platform game engine, and made a couple of improvements along the way. Here is a list of the most important features included in the update:

• Native ports for OS X and Linux

• Bloom effect (you can turn this off in the Settings menu if you don't like it)

• Steam achievements

• Adjustable image quality (may boost performance on older PCs)

• Better full screen handling and multi monitor support