Oil ends 2015 in downbeat mood, hangover to be long and painful

* Crude oil prices lost a third in value in 2015 * Outlook for 2016 remaions bleak as oversupply balloons By Henning Gloystein SINGAPORE, Dec 31 (Reuters) - Oil prices opened the final trading session of 2015 in a downbeat mood after record U.S. crude inventories reinforced concerns over a global supply glut that has pulled down prices by a third over the past year. Crude inventories in the United States, the world's largest petroleum producer, rose 2.6 million barrels last week