Add aurora borealis/australis effects

Fixes Bug 1003447

This could replace PR #175. The effect should be similar, but the images are implement as background images, removing the need for absolute positioning, z-index, etc. The bottom (southern) aurora (australis, as I've learned it is called) are also merged into a single PNG.

Since the aurora are mostly transparent and sit atop a background with some noise/grain already, I was able to get away with some lossy compression that cut the image sizes *way* down as well.