Add some more enemy/bullet types

[RCramiro]

Exactly what it says on the tin...

Right now there's only one enemy type, called DummyShip - it moves to the left at 2 pixels per frame and doesn't shoot at all. Moreover, there are currently only LinearBullets, which move across the screen in a straight line. Add some variety by creating some more enemy and bullet types.

[RCramiro]

The player will only be able to shoot bombs (defined as Bomb) and normal/charged shots (both of which are just instances of LinearBullets with different starting parameters). All the new bullet types you'll be creating will be used exclusively by EnemyShips.