Add export to dot format

[rpavlik]

It would be slick if the graph could have a method to output to dot format. You could actually just serialize all the nodes (naming them by their index or something) and then setting their label, then serialize the edges - you don't have to do it the way you would "hand-write" a dot file.

In case it helps, this is how I do a similar thing in the jconf-grapher: https://github.com/rpavlik/jconf-grapher/blob/master/jconf2dot.py