added visualizer to create dot files

Hi,

I added a visualizer to create dot files [1]. So we can see the flow as a graph to verify model visually.

Hope you like it.

To view it, save the graph to a file 'flow.dot'
```
dot -Tpng -O flow.dot && eog flow.dot.png
```

[1] http://www.graphviz.org/content/fsm

Cheers Lars

P.S. I bumped the pom to maven 3 and the compiler to java 7 as both are official deprecated.