A phase 2, Double-Blind, Randomized, Placebo Controlled, Four-Arm, Multicenter,Dose-Finding Study to Assess the Safety and Efiicacy of Three Dose Levels of AVP- 923 (Dextromethorphan/Quinidine) in the Treatment of Central Neuropathic Pain in Patients with Multiple Sclerosis, Avanir Pharmaceuticals