altered selection object so nodes and edges can have the same ID (bug occurrence in example 16 #68)
added updateCalculationNodes to the remove node function to fix bug reported by #68
fixed bug per #70
restored example 25
altered treeconstruction to fix #71
added edge.length and node.mass to docs.
added stabilized event to trigger storePositions()
experiment
added freezeForStabilization option - experimental and undocumented
updated docs for freezeForStabilization
Merge remote-tracking branch 'origin/develop' into develop

Conflicts:
	dist/vis.js
Released version 0.7.1