A randomized, double-blind, placebo-controlled study to evaluate the effects of GW679769 on Current Perception Threshold (CPT) following a single 150 mg dose (Part A) and to evaluate the effect of CPT and symptoms of overactive bladder following 28 days of repeat dosing with 120 mg GW679769 administered once daily (Part B) in patients with neurogenic bladder due to incomplete spinal cord injury