* ENH: added some utility method that builds a flat dictionnary

  * ENH: added some utility method that builds a flat dictionnary
    holding all the Dicom entries contained in the recursive structure
    of a gdcmElementSet. Refer to add FlatHashTablePrint.cxx for
    an example of usage.
    - src/gdcmDocument.[h|cxx] added BuildFlatHashTableRecurse() and
      BuildFlatHashTable() that build a flat dictionary.
    - src/gdcmElementSet.h: added a new private GetTag() accessor.
      gdcmDocument is now a friend of gdcmElementSet.
    - src/gdcmElementSet.cxx: clean up.
    - Example/FlatHashTablePrint.cxx added.
    - Example/CmakeLists.txt changed accordingly