Allow edge label fill color customization

Hello,

I was creating a graph with a non-white background with edge labels and they use fixed white backgound.

I created a backwards-compatible property for modifying this value and modified the documentation.

Let me know what you think!

Before and after example:

![Before](http://i.imgur.com/95391sY.png)

![After](http://i.imgur.com/9c2eIM3.png)