More advanced dot graph generator.

Micah,

Thanks a lot for maintaining the statemachine gem, has been very useful in an opensource project I'm working on to simulate CNC and Robot interactions. I needed a some enhancements to the dot graph generator to handle the complex transitions I needed. I have modified the code in the generator and rerun the tests. 

The new code handles substates, default transitions, and hierarchal transitions to superstates as cluster boxes. I think this looks a bit prettier for complex state machines. The full sim can be found https://github.com/mtconnect/ros_bridge/tree/master/simulator if you want to take a look at the actual state machine.

Attached is an image of one of the state machines I'm developing.

Best Regards,
Will Sobel

![main](https://f.cloud.github.com/assets/1302665/245114/2b399430-8a77-11e2-9425-8a158d82d35f.png)