It’s said that necessity breeds innovation. During the three-month writer’s strike, Joss Whedon and Co. decided that, if they couldn’t write for others, they’d write for themselves. And produce it. And release it. They created “Dr. Horrible’s Sing-Along Blog,” a 40-minute, three-part super-villian musical-comedy. (Whedon fans are familiar with “Once More, With Feeling,” the musical [...]