Added boss, tweaked numbers, improved FPS

- added first boss (mothership)
- bullets explode when they hit players
- played with damage values, health values, etc to balance gameplay
- fixed broken background tiling
- title screen animation tiles infinitely
- explosions can now be tethered to an entity and can spawn at an offset to the entitie's X and Y values
- explosions can also be scaled up and down
- improved performance when masisve amounts of enemies are on screen