change init root to window
safer window context fix
support multi-touch
check for body first
added integrator testing sandbox
feature added velocity-verlet integrator
bug fix: multiple calls to ticker.start fixed.

ticker starts automatically
re issue #42. remember previous contacts
created better velocity verlet integrator
minor change
fix for timestep rounding errors
fix for pubsub priorities
fix: circle collisions without overlap were faulty
fix for #76 polygon collision detection

contact points were being reported incorrectly
added collision sandbox
fix sweep-prune not resetting between steps
apply propper friction for contacts
added hook for "beforeStep"
added sweep-prune interval draw
fix re #76. sweep prune had wrong aabb dims