Bug in the strongly connected component algorithm

[mmuesly]

Due to incorrect backtracking on the stack, wrong components are put together using TarjanSCCVisitor.

The initial bug triggering example was the following graph:

nodes: a,b,c,d,e

edges: a -> b b -> c b -> e c -> d d -> b

I expected the following three sccs: {a}, {b,c,d} {e}, but got {a}, {b, e}, {c, d}.

[mmuesly]

I suggest pr #22 as solution.

[mtf90]

PR #22 merged