Pull request: Added package org.neuroph.contrib.graphml

Support to export trained neural networks as a graphml file (http://graphml.graphdrawing.org/). 

This format is useful to transfer network architecture from neuroph to other frameworks (e.g. Gephi (https://gephi.github.io/) and igraph (http://igraph.org/redirect.html) ). 

Example code can be found under org.neuroph.contrib.graphml.Example.java