Adding some tank art.

tank.png - The image very similair to the original. Notes:

* Two colors of tanks, easily changable via tank.xcf
* First four frames are treads moving together
* Last four frames are treads moving in opposite directions
* Turrets have had sizes changed from 24x77 center offset 12,64 to 32x64 center offset: 16,48. This will have to be changed in the code to reflect the proper center axis.

tank.xcf - The full raw tank. Contains:

* Three layers (turrent,body,treads)
* Shading
* Shadows
* Interchangable colors
* Camo
* Lineart

tank.readme.md - Information about the author and the license that these images fall under (MIT)

Credits in game would be appreciated!