An exhausted but gung-ho Scott Brown vowed yesterday to win his insurgent campaign for U.S. Senate - while reassuring voters that they will only be sending him to Washington for twoyears. “This is like a trial period,” he said. “People have the opportunity to send me down there for a couple years. ’Cause I’m up for re-election soon thereafter. So what have they got to lose?” Sitting down with the Herald editorial board, the Republican state senator from Wrentham noted Democratic Attorney General Martha Coakley’s light schedule of events. “While Martha’s taking kind of a break, I’ve been everywhere,” he...