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
Added some useful new settings

"enableCamera" makes possible to disable any camera position update (default to true).
"enableHovering" makes possible to disable hovered nodes (default to true).
"rescaleIgnoreSize" makes the rescale middleware ignore node sizes, useful when the node sizes are dynamically updated (default to false).
Merge branch 'draft-v1.0.0' of https://github.com/jacomyal/sigma.js into draft-v1.0.0