GLOBAL MARKETS-Stocks up as weak US data tied to weather; dlr drops

* U.S. retail sales unexpectedly fall; jobless claims rise * U.S. stocks rise; Nasdaq up for a sixth session * European stocks dip * Italy's Letta announces resignation after market's close * Bond prices gain By Caroline Valetkevitch NEW YORK, Feb 13 (Reuters) - World stock markets edged higher on Thursday as soft U.S. economic data was blamed on bad weather, while the dollar fell to a two-week low against the euro. U.S. stocks rose, with big gains in technology sha