Apple today announced that global superstars including Justin Timberlake, Thirty Seconds to Mars, Jack Johnson and Jessie J are among the headliners at this year’s iTunes Festival in London. Running every night in September at the Roundhouse, the iTunes Festival features over 60 acts performing at the legendary venue. Performances can be watched live or on-demand by millions of iOS users around the world on their iPhone, iPad or iPod touch, as well as by music fans with iTunes on their computer or in stunning HD with Apple TV. Tickets are free for competition winners only. “This year’s iTunes Festival is the best ever with an incredible lineup of global…