Add cutoff parameter to edmonds_karp and shortest_augmenting_path
Fix issues in minimum_cut

value_only should always be True when passed to flow_func in
minimum_cut.

Node partition should be determined by reachability to the sink.
Document semantics of value_only option of maxflow algorithms

The criterion for determining minimum cuts is also clarified.
Prevent use of cutoff in minimum_cut
Merge pull request #2 from ysitu/refactor-flow

Implement cutoff and fix value_only use in minimum_cut