BEITBRIDGE, Zimbabwe (Reuters) - President Robert Mugabe dismissed his two challengers in next month's Zimbabwean elections as a lightweight and a Western puppet on Saturday. In a rally to mark his 84th birthday and launch his campaign for another five-year term, Mugabe said his ruling ZANU-PF party would win the March 29 votes resoundingly. Former Finance Minister Simba Makoni, who says he has the support of a number of ZANU-PF officials, is standing against Mugabe in the presidential contest. "He is like a frog trying to inflate itself up to the size of an ox. It will burst," Mugabe told...