'Up,' Tribune rating: **** The creative people at Pixar have become so skilled that they can take a loosely plotted story about an elderly man, a lonely boy scout, a talking dog and a flying house and turn it into one of the most popular — and best — movies of the year. A lot of justified praise has been thrown at the opening sequence of "Up," in which the arc of a lifelong love affair is laid out in five gorgeous, wordless minutes, but the rest of the movie is just as artful, using wild jungle adventure as a metaphor for one man's need to let go.