Merge pull request #92 from BU-CS673/graphics

Graphics -> Canfield
Fixed many issues w/ Graphics tests

Switching to use OpenGL ES 2.0 (emulated) in libgdx caused most graphics
tests to fail. This was necessary to allow us to use non-power-of-2
textures.
PillboxSprite throws an exception if default case reached

- Also added a TODO and removed an empty line.
GrassSprite now uses the correct texture

- Updated Sprint1 class so we can now use non-power-of-2 textures
- GrassSprite now refers to Entity rather than the concrete Grass class
(no need for anything more than Entity)
Enemy tank is now red
Update TankSprite to work with 2 tank types

- Local tank is now blue, to differentiate it from the grass (the colors
were too similar before)
The tank is now animated

Made a number of changes to the tank treads to improve their animation
appearance
Added more comments to TankSprite
Merge pull request #94 from BU-CS673/Canfield

Canfield -> Graphics