Add qunit
Update test config
Add ParticleSystem tests
Move custom assertions out of modules
Update ParticleSystem position helpers

- Enable setting position from vec3
- Add getPosition helper
Add close-enough assertions, reorder tests
Add DistanceConstraint tests, fix solving for identical positions
Update test favicon
Fix zero distance evaluation
Enable removing constraints and forces, add tests
Add Vec3 utility
Add BoxConstraint tests
Add particle weighting, refactor constraint indices

Storing indices as particle index rather than precalculated
vec3 offsets simplifies accessing both position and weight
attributes, requires less memory, and has no
[performance penalty](http://jsperf.com/cached-v-calculated-indexing).
Add tests to jshint
Merge branch 'feature/weights' into develop
Add constructor functions
Simplify force constructors
Add PointConstraint, enable pinning particles
Refactor examples to use pinning methods
Account for particle weights in forces