A Ten-Week, Multicenter, Randomized, Double-Blind, Placebo and Active-Controlled, Parallel-Group, Flexible-Dose Study Evaluating the Efficacy, Safety, and Tolerability of GSK372475 (1.5 mg/day to 2.0 mg/day) or Extended Release Venlafaxine XR (150 mg/day to 225 mg/day) Compared to Placebo in Adult Subjects Diagnosed with Major Depressive Disorder