Improve the function-type interface of bfs, dfs, and dijkstra (ticket #96)
- BfsWizard and DfsWizard have run(s), run(s,t), and run() functions,
DijkstraWizard has run(s) and run(s,t) functions.
- Set NodeMap<T> instead of NullMap as PredMap and DistMap in the default
traits classes for the function-type interface.
- Modify the related test files.
- Doc improvements.
- Bug fix in concepts/path.h.

 INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR})

 LINK_DIRECTORIES(${CMAKE_BINARY_DIR}/lemon)

 SET(DEMOS

   arg_parser_demo

   graph_to_eps_demo

   lgf_demo)

 FOREACH(DEMO_NAME ${DEMOS})

   ADD_EXECUTABLE(${DEMO_NAME} ${DEMO_NAME}.cc)

   TARGET_LINK_LIBRARIES(${DEMO_NAME} lemon)

 ENDFOREACH(DEMO_NAME)