Teen pop star Justin Bieber said he plans to 'just relax' on his upcoming 17th birthday.The singer, who turns 17 on March 1, only has a few days off before he starts his world tour so wants to celebrate his birthday quietly.He told ETOnline.com: 'I only have, like, four days, so I'm just going to relax, because I start my world tour on March 4 so just going to mentally prepare and just relax.'No