Efficacy and safety of 3 doses (0.25, 0.5 and 1mg/day) of agomelatine sublingual administration over an 8-week treatment period, in out-patients with Major Depressive Disorder. An 8-week randomised, double-blind, fixed dose, international multicentre, placebo-controlled study with parallel groups, followed by an extension double-blind treatment period of 16 weeks