force cluster example: fixed minor hull error ('hulls' --> 'hullset') + ...

...augmented example, showing distance function based on collected node/group info + click on node to expand/collapse (toggle) instead of harder-to-perform dblclick.

(fix: the force example had two var hulls (one object, one array) in a single function. It works, but is not something you want others to copy.)

Now the force-cluster example shows a graphic identical to http://bl.ocks.org/3071239