Added JGraphXAdapter

We added and implemented a fully working JGraphXAdapter with documentation, tests and example.

We took up the project from [here](http://forum.jgraph.com/questions/133/using-jgraphx-to-visualize-jgrapht/137), added some functionality and tests as well as documentation. 
Someone on the stackexchange mentioned sending this to you already - but I don't see it in the repository.

Changes are only partially made by me, the base/idea came from JeanYves Tinevez.