Version 0.5. Enemies movement & deaths

- Added a vector of enemies in the Game Scene.
- Enemies moves across the screen endlessly.
- Added a structure in the Game Loop to check all the enemies status and
hits with the bullets. If an enemy is hit, he disappears and the enemies
vector is reduced by 1. So if you kill all the enemies the vector is
empty.
- Added parameters to the constructor of Bullet and Enemy.