Merge pull request #33 from playcanvas/master

merge
Added documentation for particlesystem component.
Old particle system was deleted.
Removed 2 from ParticleEmitter2/particle2.
depthTest renamed to depthWrite in particles (that's what it actually does).
Removed unneeded code from simulation.
Fixed flickering bug when using CPU wrapping.
Yet another particle gamma correction fix + replaced old unnecessary slow code.
Merge pull request #121 from guycalledfrank/particle-system-docs-fixes

Particle system docs & fixes