add testing assets within assets directory structure

[andydelso]

All images were made using gimp.

Includes the following in both 16 and 32 pixels:

• enemy
• player
• hard obstacle
• soft obstacle
• item
• ground
• blank sprite setup for gimp

Work for #22

[BedfordWest]

Good deal. Couple of things:

1. We probably need to add the .xcf files to gitignore - don't think we need them here?
2. Can you add an orientation arrow on the player and enemy sprites? We need some way to verify the left/right/away/toward facings are working properly.

[andydelso]

issue 1 resolved, working on 2

[BedfordWest]

@ddaypunk06 :+1: let me know if the orientation thing is unclear. Basically we just need a horizontal and vertical indicator, so something as simple as a dot/line/arrow/etc. on the left or right and one on the top or bottom would do fine.