Implement Graph#bipartite_sets.

Here's an implementation of an algorithm that splits vertices of a bipartite graph into two sets. It can also be used to check if a graph is bipartite or not.

Before releasing a new version of RGL I'd like to implement one more algorithm – Khun's algorithm for finding a maximum matching in a bipartite graph. I'm not quite sure when I'll have time to work on it, but I hope I'll get to it soon.