Merge pull request #22 from tingnan/develop

fix mac os link error  (gcc 4.9)
Demonstrates ChParticleProcessor
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop

Conflicts:
	src/demos/irrlicht/demo_emitter.cpp
Fix path to unit_IRRLICHT headers.
Make all `ChLinkEngine` accessors const functions.