Minor fix in sigma.parsers.json

When called with only a path and a callback, the callback was actually not called at all. Now it is.
Big improvement: Canvases now merged when possible

From now, nodes canvas and labels canvas are the same. Also, the edges canvas is merged to the nodes/labels canvas if the setting "batchEdgesDrawing" is set to false. That should greatly improve the canvas renderer performances.
Merge branch 'draft-v1.0.0' of github.com:jacomyal/sigma.js into draft-v1.0.0