Add dot graph generation from a summing bird graph.

Use with
  import com.twitter.summingbird.viz.BaseViz

    val writer = new PrintWriter(new File("/tmp/output.dot"))
  BaseViz(node, writer)
    writer.close()
Push the graph val back private in Dependants.
Add a nodes function to return the set of keys in the Dependants graph.
Pull the VizGraph case class out of BaseViz, move access of the graph from a local variable to using a dependentState variable with the nodes and getDependantsOf accessors.
Merge pull request #223 from twitter/feature/add_dot_graph

Add dot graph generation from a summing bird graph.