Add support for more file formats

[qiemem]

There are some common, simple ones that would be easy to add support for and would greatly enhance nw's ability to use existing data.

Possible additions:

• GUESS's gdf
• Pajek's net
• ucinet's dl
• Simple csv representations
• Graphviz's dot.
• GML
• GEXF

Gephi gives a good comparison of various formats: https://gephi.github.io/users/supported-graph-formats/. This includes examples and basic specs.

[qiemem]

This site contains a bunch of great sample networks: http://snap.stanford.edu/data/index.html#web

Most use a simple format where each line is an edge with the format "source-node-id dest-node-id" and # are used to denote comments.

[qiemem]

Note that NameGenWeb, what we were previously using for generating GrahpML representations of people's facebook networks, appears to be defunct: https://apps.facebook.com/namegenweb/. netvizz, however, is very much active and uses GDF. Thus, I am inclined to implement GDF first.

[qiemem]

Fixed in latest release.