Adding function duplicated2
cf. http://joseph.larmarange.net/?Rechercher-et-visualiser-les
Function na.rm  cf. http://joseph.larmarange.net/?Manipulation-de-donnees-quelques
Function rm.unused.levels cf. http://joseph.larmarange.net/?Manipulation-de-donnees-quelques
Merge pull request #27 from larmarange/issue17

New functions : duplicated2, na.rm, rm.unused.levels