Feature auto layout

Use jgraphx layout capabilities from the PIPECore repository to auto-layout Petri nets.

It's quite experimental and basic but there is a form to help provide Petri net layout.
We can go from tangled Petri nets like this:
![screen shot 2014-06-23 at 14 48 18](https://cloud.githubusercontent.com/assets/850346/3358333/13d96c02-fadd-11e3-9bfb-08b070766900.png)
![screen shot 2014-06-23 at 14 48 22](https://cloud.githubusercontent.com/assets/850346/3358334/13dce4fe-fadd-11e3-8c75-31efa9af0431.png)
To this:
![screen shot 2014-06-23 at 14 48 27](https://cloud.githubusercontent.com/assets/850346/3358335/13e0a29c-fadd-11e3-9e87-ee8c1279d196.png)