Add .psd file support to SpriteBuilder.

SpriteBuilder projects can now accept Photoshop .psd files. It occurred to me that this might be really, _really_ easy to support since everything is built on top of ImageIO. It was even one of the most straightforward changes I've made all week! \o/

Artists *love* this feature of Unity since they don't need to export .png files by hand and save their .psd files somewhere else. I've seriously been told this by at least a dozen other people that it's one of their favorite Unity features.