By Marc Jones LONDON (Reuters) - European shares and bonds held on to recent gains on Thursday while gold and the euro gained, after data suggested the U.S. Federal Reserve may have to leave its stimulus program in place for a little longer. European shares on the FTSEurofirst 300 opened flat after a 3.2 percent jump in the previous two days, while the bonds of governments from Germany to Greece continued to claw back ground lost last week. The market tone improved overnight after a surprisingly sharp downward revision to first-quarter U.S. ...