Violent movies and real-life murders involving guns and young people could be linked, a new study found.  The rising number of violent gun scenes in blockbuster movies over the last two decades closely mirrors a surge in firearm-related murders among Americans aged 15 to 25 over the same period, concluded researchers at the Annenberg Public Policy Center at the University of Pennsylvania.  The analysis, which examined over 25,000 random, five-minute segments from the top 30 movies and TV series