By Oliver Holmes and Hamdi Istanbullu BEIRUT/ISTANBUL (Reuters) - Fifteen Syrian students were killed when rebel mortar shells hit a Damascus University canteen on Thursday, state-run news agency SANA said, as attacks intensified in the center of the capital. The Syrian Observatory for Human Rights, an opposition monitoring group, said a mortar killed 13 people at the university, without saying who fired the bombs. Other activists confirmed the attack but no opposition group has denied or claimed responsibility. ...