Hoping to juice up its sales, the dairy industry is making big changes in the looks of the plastic jug, created half a century ago.New on the market are a pitcher-like jug so easy to pour from that a 4-year-old can use it -- without spilling -- and jazzy single-serving bottles of flavored milk aimed at teen-age boys that would replace the half-pint cartons that are the staple of school cafeterias.