Added Direction object, which takes two components (x, y) and creates a
normalized vector in the same direction (length 1). Applied the
normalization to stick inputs for movement and firing, resulting in
diagonals no longer moving faster. Calls to the Bullet superconstructor
now take an additonal "offset" parameter that specifies how far from the
center they should spawn. Stamina/fire rate values modified slightly to
produce more intuitive results.