Today’s kids probably think rappers have always been purveyors of commercial goods. 50 Cent has Vitamin Water. Diddy and Burger King are partners. Common probably even folds every shirt at The Gap. However, there was a time when rappers weren’t viable salespeople because their music wasn’t reaching a broad segment of the population. In 1990, a Bay Area entertainer who went by the moniker MC Hammer helped set rap on track to be a cultural......