Refactored drawEdges, drawNodes, drawLabels

These three parameters are no more numbers but booleans. In practive, only the edges are sometimes drawn as batches.
So instead of using drawEdges === 1 or drawEdges === 2, a new parameter, "batchEdgesDrawing", determines wether or not edges must be batched.

Also updated the related example.
Added sigma.classes.graph.read method

This methods takes a graph object as input: An object with an array of valid node objects and an array of valid edge objects, associated respectively to the keys "nodes" and "edges".

Also added the related unit tests, and called this method in sigma constructor if a graph is given in the configuration.
Updated default settings

First, the edges are now by default always drawn, and with no batching.
The min and max size values for nodes and edges are now set such that displaying any graph looks pretty nice.

Also updated the examples with the new default settings.