drawCubicBezier no longer fills in bezier curves.

Test case: "Node: Draw > Test DrawNode"

I made this change so I could draw bezier curves in a Cocos2d-js project. It's not a perfect change, but I thought I'd submit a pull request in case you guys find it useful.

Known issues:
As you can see, I'm calling drawSegment() with a constant 0.5f line width. This line width should probably be passed in as an argument to drawCubicBezier, but since 1) that would change the interface of the function and 2) I wasn't for sure how to update the js-bindings, I was thinking it would be best if someone more knowledgable makes that change.

drawQuadBezier could probably benefit from a similar fix.

Unfortunately I don't quite have the time to perfect this change, but hopefully it's a useful start :D