Clean up

[fernandojsg]

• Cleaning up enemy code
• Introducing poolHelper
• Added ASHOOTER namespace to use it instead of AFRAME.
• Adding 2 basic enemies: enemy0, enemy1. Each enemy uses one of the movement that @ngokevin implemented. So we decopule the enemy component from the enemy behaviour implementation (Using registerEnemy). Right now a constant number of enemies are created on the enemy system init, and when you kill one of them, another one appears, so you have always the same amount of enemies on the scene. The idea is to add a wavesManager that will take care of this logic (https://github.com/fernandojsg/a-shooter/issues/27) and will create waves with each kind of registered enemies.