Add curve & arrow edge rendering

[ssidorchik]

In previous library version there was ability to render edges both curve with arrow head. I have restored this feature for the recent version. The edge type can be chosen by specifying 2 parameters: edge.type & edge.head. At the moment there are four edge types:

• straight

edge.type = 'def' || '' || undefined
edge.head = 'def' || '' || undefined

• curve

edge.type = 'curve'
edge.head = 'def' || '' || undefined

• straight with arrow head

edge.type = 'def' || '' || undefined
edge.head = 'arrow'

• curve with arrow head

edge.type = 'curve'
edge.head = 'arrow'

[jacomyal]

Thanks a lot for the pull request, I looked at it and it looks great. However, I have a small problem with it:

With your modifications, if I set an edge type to 'arrow', it will be displayed as a straight arrow with the WebGL renderer, but as a simple line with the Canvas renderer. I would prefer to keep the edge renderers as a one-level object, with 'def', 'arrow', 'curve' and 'curvedArrow', such that the API works the same for WebGL and Canvas, edges and nodes.

[ssidorchik]

At first thought that edges line and head rendering can be separated, but then realize that it's hard to accomplish. So, having one parameter is better than two and also it's consistent with WebGL renderer.
Returned the edge rendering selection mechanism to default.

[jacomyal]

I just merged it, thanks a lot!

[antonioaltamura]

i dont get if with this i can choose the arrow orientation: source or target.
can i?

[sebastialonso]

Is this documented somewhere?

[walkerlala]

the default of minArrowSize should not be 0 !