New GitHub credit card icons

Originally I set out to only modify the icons of Visual Idiot and Dan Dean, but I ended up overhauling them pretty extensively. I started with retina versions, then scaled them down and tweaked things as necessary for the 1x versions. The small versions need a little more work I think, but these are a decent first pass.

Thanks to @Visual Idiot and @dandean for the freebies I riffed off. Much love