Default hover node renderer now checking type

From now, the default hover node canvas renderer will check the node's type and use the related canvas node renderer to renderer the node hover the label's background.
Added a custom node renderer example

This is a basic example on how to develop a custom node renderer. In this example, the renderer will display an image clipped in a disc, with a border colored according the node's "color" value.

If a node as the value "image" to its attribute "type", then it will displayed with the node renderer "sigma.canvas.nodes.image", with the url being its "url" value.

IMPORTANT: This node renderer just works with the canvas renderer. If you do want to display images with the WebGL renderer, you will have to develop a specific WebGL node renderer.