Implement Graph#bipartite_sets.
Do not use dotty in tests
Fixed issue #13 "Implicit graph example fails"

This was only a problem with ruby >= 1.9 and only with the special example
module_graph. Made Graph#to_s more robust (but also more inefficient :-().
Fixes the image paths in the README
Merge pull request #14 from matiaskorhonen/patch-1

Fixes the image paths in the README.
Defined hash also for DirectedEdge
Prepare release 0.5.0

- Removed references to rubyforge
- Made compatible to ruby 1.9.1
Removed link to SWiK (apparently gone away)