Make Graphviz graph attributes work.
Renamed to setLayoutBy + some documentation.
Merge remote-tracking branch 'clemens-tolboom/graphviz-graph' into graphviz-graph
Implement LayoutableInterface for Graph instead of GraphViz
Additional helpers for default layout
Merge pull request #22 from clue/graphviz-graph

[WIP] Add LayoutableInterface + implementation for Graph