When a writer crafts a cliffhanger he or she either puts a character in jeopardy -- if a gun is fired on a TV show, you can bet that's where they'll cut to commercial -- or creates a provocative mystery. All serial dramas use these techniques, but none has ever captured the imagination of the nation as did the 'Who Shot J.R.?' episode of 'Dallas,' 30 years ago this week. In 1979, 'Dallas' had...