Add support for community detection, based on edge centrality
Community structure detection based on the edge betweenness centrality of the network, as defined in [http://www.pnas.org/cgi/content/abstract/99/12/7821 M. Girvan and M. E. J. Newman: Community structure in social and biological networks, Proc. Nat. Acad. Sci. USA 99, 7821-7826 (2002).].