added input constraits and error checking. cleaned up code (fixed magic numbers, 80 ln margin, line spacing/indentation, deleted commented out code)
Merge branch 'master' of https://github.com/nancywong/ComputerTowerDefense into nancy
moved back to before encountering vector error, still have constraints but not projectile directions
working checkpoint
Merge pull request #6 from nancywong/nancy

Nancy