add horse walk-cycle sprite

[crisner]

Added up and down sprite. Also included left and right animation sprite in the same file. Included two png files of different sizes and an svg file named horse-walk-cycle-sprite.png, horse-walk-cycle-sprite-small.png, and horse-walk-cycle-sprite.svg.

Issue #54

[caseydierking]

@crisner , This is ridiculous man. Awesome work. I'm impressed at how accurate the up and down horse sprite is, given you had nothing to work off of. 💯