Fix multiple registration of nodes, add events
Update dragNodes example
Update dragNodes README
Fix a couple of issues in dragNodes

Ensure 'this' is related to the object.
Add compatibility with edgequadtree.
Enable drag if graph size=1.
Add killDragNodes and refactor the code
Fix undefined sigma
Prevent killDragNodes if not instanciated
Fix excessive refresh
Merge remote-tracking branch 'origin/fix-drag-nodes' into plugin/active-state
Update Gruntfile