A Multicentre, Randomised, Double-Blind, Double Dummy, Parallel Group, 104 Week Study to Compare the Effect of the Salmeterol/Fluticasone Propionate Combination Product (SERETIDE) 50/500mcg with Tiotropium Bromide 18 mcg on the Rate of Exacerbations in Subjects with Severe Chronic Obstructive Pulmonary Disease (COPD)