http://www.migapro.com/circle-and-rotated-rectangle-collision-detection/
http://gamedevelopment.tutsplus.com/series/understanding-steering-behaviors--gamedev-12732
http://www.gamedev.net/page/resources/_/technical/artificial-intelligence/generalized-platformer-ai-pathfinding-r3924
http://www.gabrielgambetta.com/fast_paced_multiplayer.html
http://higherorderfun.com/blog/2012/05/20/the-guide-to-implementing-2d-platformers/
http://www.redblobgames.com/grids/hexagons/#line-drawing