A Randomized, Double-Blind, Double-Dummy, Parallel Group, 12-Week Comparative Trial of Salmeterol/Fluticasone Propionate Combination Product 50/100mcg Twice Daily Via the DISKUS® Inhaler Versus Oral Montelukast 10mg Once Daily in Adolescents and Adults with Persistent Asthma, Trial ID SAS40021 | GSK