Bipartite stochastic block model community detection algorithm

This pull request contains an implementation of the algorithm described in "Efficiently inferring community structure in bipartite networks" by Larremore et. al. (http://arxiv.org/abs/1403.2933)

Essentially, the algorithm takes the classical stochastic block model approach, and extends it to work well with bipartite networks. 

This implements `igraph_community_bipartite_sbm.c` along with basic documentation and linking. I also updated the test suite with a toy example, but haven't written a suitable test yet. 

Let me know what I should change to make sure that everything is in line with the igraph standard.