CRAWFORD, Texas (Reuters) - President Bush (news - web sites) on Saturday brushed aside calls he pull political ads that use images of the devastated World Trade Center to show him as a strong leader in troubled times. Despite pressure from families who lost relatives in the Sept. 11, 2001, attacks, Bush said he "will continue to speak about the effects of 9/11 on our country and my presidency." "I have an obligation to those who died. I have an obligation to those who were heroic in their attempts to rescue (the victims). And I won't forget that obligation," Bush...