Fixed FindGUROBI for empty GUROBI_ROOT_DIR
CompleteGraph: Added Iterator inheritance from std::iterator and fixed types
CompleteGraph: Added operator- to support std::distance, added corresponding compile-time tests
Cleanup
Made pointer operator private for Vertex/Edge iterators
Iterator inheritance cleanup
Included AdjacencyType in all classes
Used typedef std::iterator as Base
Merge pull request #8 from kalofoli/iterator_fixes

correcting the iterator scheme of CompleteGraph and Graph