Replace pygraphviz with graphviz

[aleneum]

Since there are some issues with pygraphviz which makes the graph feature more or less unusable on Windows, I implemented an alternative based on graphviz. graphviz requires a Graphviz installation available in PATH. Currently, it is part of the dev-graphviz branch and I consider merging it into next-release for 0.7.0. Feel free to comment whether you consider that a valid option or not.

[aleneum]

In 0.7.0 we will test the parallel support of graphviz AND pygraphviz. As both libraries work differently, GraphMachine will not support editing Graph objects directly any longer. The Graph interface should be equivalent but is also kept simple. More sophisticated customization has to be done on AGraph or DiGraph objects directly (Graph.fsm_graph). However, the look and feel of graphs can be altered via GraphMachine.(hierarchical)_machine_attributes and GraphMachine.style_attributes and does not require fiddling with Graph attributes.