Particle systems
Merge branch 'master' into pf-particle-nits

Conflicts:
	chp04_systems/NOC_4_02_ArrayParticles/sketch.js
	chp04_systems/NOC_4_03_ParticleSystemClass/sketch.js
	chp04_systems/NOC_4_04_SystemofSystems/sketch.js
	chp04_systems/NOC_4_05_ParticleSystemInheritancePolymorphism/sketch.js
	chp04_systems/NOC_4_07_ParticleSystemForcesRepeller/sketch.js
Merge pull request #22 from shiffman/pf-particle-nits

Particle systems code formatting