Multiplayer, powerups

- added core networking code
- made the game multiplayer friendly
- explosions updated to support textureString (the game loads the image from the string: it is good for sending entity animations to clients)
- added getNearestPlayer()
- added a gamescreen for clients and a gamescreen for hosts to separate their functions
- fixed bug where the game would freak out after a certain amount of waves
- added several powerups like HP boost, DMG increase, etc (1 in 5 chance that they spawn on enemy death)
- slight texture changes
- improved how the game increases in difficulty
- improved screen shaking