Feature multiheads scc

Hi Eric, 
Here is a patch to support multi-headed directed graph when computing strongly-connected components with StronglyConnectedComponentsFast. Am I missing a reason explaining the current limitation ?

I checked the Tarjan's algorithm, and imho there is no limitation such as the number of head in the graph. The Tarjan's doesn't even requires that we identify heads of the graph.

I also fixed the identation in the source file.

Regards.