MEXICO CITY (Reuters) - Mexico's conservative presidential candidate, Felipe Calderon, snatched a razor-thin election victory on Thursday, but his leftist rival vowed to fight the result in the courts and on the streets. The Harvard-educated Calderon had an insurmountable lead with 35.87 percent of the vote and only a few dozen polling stations pending. Mexico's Federal Electoral Institute said it would declare the winner on the basis of these overall results. Andres Manuel Lopez Obrador, a combative left-wing former Mexico City mayor, was 0.57 points behind Calderon and angrily claimed the vote was plagued with irregularities. He pledged to fight...