A Randomized, Double-blind, Placebo-controlled, Parallel, 26-Week, Phase 3 Study of 2 Doses of an Alpha-7 Nicotinic Acetylcholine Receptor Agonist (EVP-6124) or Placebo as an Adjunctive Pro-cognitive Treatment in Schizophrenia Subjects on Chronic Stable Atypical Antipsychotic Therapy