 A Randomized, Double-Blind, Parallel Group, 52-week Study to Compare the Effect of the Fluticasone PropionateSalmeterol DISKUS Combination Product 25050mcg BID with Salmeterol DISKUS 50mcg BID on the Annual Rate of ModerateSevere Exacerbations in Subjects with Chronic Obstructive Pulmonary Disease COPD 