Twenty new works by Michelangelo, previously little-known or of uncertain authenticity, have now been linked to the Renaissance artist, according to a new study reported in Friday's Il Messaggero newspaper.     The discoveries are the alleged result of the work of independent Roman researcher Valentina Salerno, who in her study "Michelangelo Gli ultimi giorni" (Michelangelo the Last Days) reconstructed the final period of the artist's life by comparing dozens of documents from various Italian