Weight loss is hardly a laughing matter here in the Obese Nation,where two-thirds of adults are overweight or worse.But the newest"aha!"breakthrough in the battle of the bulge just might prompt a snicker. One can lose weight by laughing.No joke.Purposeful laughing is gaining a following.Thousands of laugh clubs worldwide invite people to,well,laugh out loud together.A hybrid branch of psychology called"laughter therapy"is finding its way into hospitals,and a new exercise movement called ...