Created Physics engine

TestEntity tests ballistic trajectory. Sprites can be reset to the 1st
frame. Improved key detection for PlayerSprite and animation as well.
Need to figure out a way to make sure velocity is consistent across all
frame rates, presumably with delta logic.
Loop improvements

Fixed step physics and logic updating. Variable step rendering. Fixed
step sprite advancement.
Merge branch 'master' of https://github.com/Im-Laodicean/you-re-trash