Support for edge multiplicities in brandes_betweenness_centrality

[Ticket #9935](https://svn.boost.org/trac/boost/ticket/9935)
Implemented support for edges multiplicities in betweenness calculation. As explained in the ticket, the algorithm variation is taken from Ulrik Brandes' paper [On variants of shortest-path betweenness centrality and their generic computation](http://www.inf.uni-konstanz.de/algo/publications/b-vspbc-08.pdf), section 3.8, algorithm 11. Please note that this paper has a mistake on this algorithm, as explained in [Brandes' publications web page](http://www.inf.uni-konstanz.de/~brandes/publications/) - this patch implements the corrected algorithm.