Feature/mover

Implemented enemy subclass Mover which moves from a fixed point in a specified direction for a specified length and then reverses its direction and travels backwords to the point and the same length again in the opposite direction.

Implemented Directions which can take 8 different directions. 
 - Left, right, up, down
 - any useful combination: left-up, left-down, right-up, right-down

Fireball is still missing 4 directions, so the game crashes when walking diagonally and then shooting a fireball...