By Nidal al-Mughrabi and Maayan Lubell GAZA/JERUSALEM (Reuters) -Israel and the Palestinian Islamic Jihad militant group declared a truce late on Sunday, raising hopes of an end to the most serious flare-up on the Gaza frontier in more than a year. Israeli forces pounded Palestinian targets through the weekend, triggering rocket attacks against its cities, which largely tapered off by the time the truce came into effect at 23:30 (20:30 GMT). It was announced in separate statements by Islamic Jihad and then Israel, who both thanked Egypt for mediating the ceasefire. The three-day clashes echoed…