Implementation of Gabow's algorithm

I added a new class with the implementation of the Gabow's algorithm for strongly connected components, that could be used as an alternative to the StrongConnectivityInspector (using Kosaraju's algorithm).

It follows Path-based depth-ﬁrst search for strong and biconnected components by Gabow 
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.121.839