extent KeyStatus to support wasd keys too
support moving two players
shoot with UP key, drop S key support
now second player can fire too
spawn two players
Hud no longer knows about the ship
add number_of_lives variable

Now we loose when all ships are destroyed and no more extra lives
left.
add id to differentiate between players
don't use player's one controls for player two and vice verse
enable multikey support i.e. when several keys are pressed
allow players to move at the same time
allow players to shoot at the same time
don't limit number of rockets, limit by reload time only
different sprites for player1 & player2
reload rockets faster