Rapper/crooner T-Pain warned the audience that his performance at The Rave would be more than just a concert: It was going to be a full-out party, he said, as he shook his head-full of blond-dipped black dreads, which were heaped into a rear ponytail. T-Pain has mastered the art of creative live oration, marrying many songs to voluptuous choruses accompanied by acoustic guitar. Last Wednesday, when he delivered his 2005 debut single, “I’m Sprung,” about the euphoric feeling of falling in love . . .