graph_to_eps mission accomplished.
- lemon/graph_to_eps.h header created
- lemon/bezier.h: Tools to compute with bezier curves (unclean and undocumented
interface, used internally by graph_to_eps.h)
- demo/graph_to_eps_demo.cc: a simple demo for lemon/graph_to_eps.h

 ./preflow_demo_leda < $1

 ./preflow_demo_boost < $1

 ./preflow_demo_jacint < $1

 ./preflow_demo_athos < $1

 ./edmonds_karp_demo < $1

 ./edmonds_karp_demo_alpar < $1

 ./edmonds_karp_demo_boost < $1