A Randomized, Double-Blind, Parallel Group, 52-week Study to Compare the Effect of the Fluticasone Propionate/Salmeterol DISKUS® inhaler Combination Product 250/50mcg twice daily (BID) with Salmeterol DISKUS® inhaler 50mcg BID on the Annual Rate of Moderate/Severe Exacerbations in Subjects with Chronic Obstructive Pulmonary Disease (COPD)