Add Robins and Alexander bipartite clustering

This pull request adds Robins and Alexander (2004) bipartite clustering coefficient computation. It is defined as four times the number of four cycles divided by the number of three paths. This is a commonly used measure in social network analysis literature. I've added tests and also tested the results against the fantastic statnet R suite. 

I renamed the current `clustering` function to `latapy_clustering`  but added `clustering = latapy_clustering` for backwards compatibility. There are other interesting bipartite clustering measures, I plan to implement Opsahl (2012) in the future. 

Robins, G. and M. Alexander (2004). Small worlds among interlocking directors: Network structure and distance in bipartite graphs. Computational & Mathematical Organization Theory 10(1), 69-94.

Opsahl, T (2012). Triadic closure in two-mode networks: Redefining the global and local clustering coefficients. Social Networks 34
http://arxiv.org/abs/1006.0887