Add a Particle System to simulate rain.

[mcgivrer]

Create a new ParticleSystem, a GameObject implementation, to manage a bundle of particles.
A Particle is an interface, it will be implemented into some new Class, like Rain and Drop*

• A ParticleBehavior is an interface and will be implemented to create, animate and render those particles. A RainBehavior can be used to produce Rain effect.

The ParticleSystem will implement a chained API to set its attributes values.