Add a Particle System

This add a simple but flexible (and some powerfull) Particle System to melonJS (see the examples)!

And it eliminated the oldest melonJS open ticket #9 ;)

- Add a ParticleEmitter Object
- Add a Particle Object
- Added the Particle in the object pool
- Create a example with several particle emitter behaviors
- Adjusted copyright year and contributors in some files (CHANGELOG, README, AUTHORS)

Some tips:

- For best performance, the particles require a image (sprite), not using native canvas draw (like arc)
- The emitter has a parameter to enable Texture Additive (by globalCompositeOperation = lighter)
- I not figure out a way to build test unit on it :(

For the future:

- The particles can collide with the game world (after the new collision mechanism from ticket #103)

Thanks!