Dot output format support

Hello Rudolph,

I have added graphviz/dot output support to sbt-dependency-graph.
I also fixed two things I consider bugs:
4bc9e3f7dfc88d988a424e  - renamed Version.unapplySeq to Version.unapply
25a996752c159500049 -  removed akka-actor from sbt-test ignoreScalaLibary

Let me know what you think.
Cheers and merry Christmas,
Leon