Automate extracting PNG images from "Apple Color Emoji.ttf"
Extract and optimize PNGs from "Apple Color Emoji.ttf"

Commands used:

    $ rake images:extract
    $ imageOptim -a -d images/emoji/unicode/

This required both ImageOptim and ImageAlpha to be installed as OS X apps.
Update test for expected identical emoji checksums
Simplify special case for "black_medium_square" in checksum test
Run the rest of the PNGs through ImageOptim

Command used:

    $ imageOptim -a -d images/emoji/
Tweak extractor.rb code style