Add support for Arduino Leonardo.

WIP: Leonardo support.

I'm not sure this is 100% right; I have been testing with an A-Star Micro (http://www.pololu.com/product/3101) since I don't have an actual Leonardo. I think I may not have the pin mapping exactly right since I haven't successfully gotten it to blink, though it does upload. Anyway it needs a bit more work, but I thought I'd throw this out there as a sanity check.