Changed method by which zombies travel toward the player. They now travel along an arc tangent like the particle class (which was also fixed because it was using the wrong Math method for X and Y respectively). Also, zombies now spawn via threads. There is a thread for each zombie type. There are also a new type of zombie, the zombie dog, which move fast, and do a lot more damage. The sprite used for them is temporary, as I don't think it looks very good from above when rotated. Re-factored code to reflect these changes.