Reorganize header files (Ticket #97)

In addition on some places the DefaultMap<G, K, V> is replaced with
ItemSetTraits<G, K>::template Map<V>::Type, to decrease the dependencies
of different tools. It is obviously better solution.

 project (LEMON)

 enable_testing ()

 add_subdirectory (lemon)

 add_subdirectory (demo)

 add_subdirectory (test)