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`
Convert sprite.png to 8-bit.

11840 -> 3078 bytes
Use green color for the donate icon.
Hide the downloads navbar link on extra small devices (<768px).

Should do the job until a better solution is available.