Added try/catch for control WebGL options in "Hard Environments"
sigma.utils.zoomTo() almost done. Only left the inital checking of the animation settings and the documentation of the function.
sigma.utils.zoomTo() done! Also the three events when it is possible to use it are using it now.
Fixed #241: Improved canvas renderer

The canvas renderer no more uses two different canvases to draw edges and nodes when the `'batchEdgesDrawing'` setting is truthy:
It uses `context.globalCompositeOperation = 'destination-over'` instead.
Fixed #244: (min|max)(Node|Edge)Size as strings

The settings minNodeSize, maxNodeSize, minEdgeSize and maxEdgeSize now work both as number or string.