Network Flow Algorithms

Yes, I'll put support for this, since it's a fairly simple wrap around
BGL, and is quite important code. I'm doing some other modifications at
the moment, so this will have to wait a little bit. If you want, you can
add a ticket for it in the website, so this can be tracked more easily
(you will get a nice email telling when it's done and so forth :-).

Cheers,
Tiago