Add Layout to the Visualization

[yilinxia]

Hi there, recently we are using the JSNetworkX to construct visualizations. By looking at the documents and the code (some referenece website don't work now), we haven't found a proper way to set up the layout for visualizations. #23 did briefly mentioned that the layout function is somehow under constructiuon back in 2014, and we would really appreciate it if you can point us to website or documents on setting up the layout for JSNetworkX. Thank you so much!