Enemy framework

I have made the enemy framework. This class should work fine now, but I
have to make a enemies manager because if graphics of the enemies have
to be loaded everytime a new enemy is created, my pc would blew up.
Improved enemies manager

Enemy class is now used as base class to make different types of
enemies.
Enemies has now a movement system which uses random speeds for movement.
Small changes on movement

I have modified a bit the generation of the speeds.