match github emoji and add retina support

This adds retina support, matches the emoji set to github's, and improves the overall size of the emoji. 

I manually added the @2x icons to the directory and removed the .png from ` <key>` Emoticons.plst, so it seems like these would need to ba accommodated for in the build step. If you're interested in the change this PR makes, I can look at that more deeply.