Recreate the sprite to include retina images.

Replace the paypal image with a Font Awesome icon for the time being.

Command used to create the sprite:
`glue input output --margin=10 -a vertical --retina`

Fixes #24.

Would be nice if we had a Paypal Font Awesome icon but apparently this won't happen [any time soon](https://github.com/FortAwesome/Font-Awesome/issues/1009)...