Leapfrog integration implemented.
Incorporated global time step into leapfrog integrator.
Added new keycode R.  Reverses direction of flow of time.
Added function to see if two particles are gravitationally bound.  Untested.
Got leapfrog separate; working, adding fields to fix checkclock.
Finished math for collide_glom.  Think I fixed checkClock
Moving methods off of global scope.

Moving leapfrog and collide_glom onto Physics.
Merge branch 'leapfrog'

Conflicts:
	particles.js