add method simple_cycles

Graph-Tool needs a method to find all cycles in a graph. A common and one of the most efficient ways to do this is via Johnson's Algorithm / Tarjan's Algorithm. There are two variants - the original variant is a recursive version and a more recent version uses Nuutila's modification to provide a non-recursive version. As a reference, a Network X version of this algorithm can be found at http://networkx.lanl.gov/_modules/networkx/algorithms/cycles.html#simple_cycles