remove sprite knowledge from animation

The animation should be independent of a sprite. This will allow it to
be used for other interesting things later, and in general is a better
composition.
DRY up adding animations
better variable names
remove MGQ-specific stuff from Animation
move Animation to new Foundations asm
fix csproj to use new asm name
comments for Animation
README for Foundation asm
better, shorter name
add general sprite to Foundation
derive PlayerSprite from Sprite
code doc for Sprite
Merge pull request #8 from half-ogre/half-ogre/6-monogame-foundation

Move Animation and Sprite into a new Foundation assembly