isEmpty = function(obj) {
		for (var k in obj) {
			if (obj.hasOwnProperty(k)) {
				return false;
			}
		}
		return true;
	}
Starts the simulation of the layout in use.

Note that in case the algorithm is still or already running then the layout that's in use
might silently ignore the call, and your optional <code>done</code> callback is never executed.
At least the built-in ForceDirected layout behaves in this way.

@param done An optional callback function that gets executed when the springy algorithm stops,
either because it ended or because stop() was called.