The German television company EM.TV agreed Monday to buy The Jim Henson Co., creator of the Muppets Show and all the "Sesame Street" characters, for $680 million in cash and stock. The deal culminates a 42-year run during which Los Angeles-based Henson Co., largely under the guidance of its creator and namesake, became one of the world's best-known independent children's entertainment production companies, creating Kermit the Frog, Miss Piggy, Big Bird and other popular characters