The work of B.D. and T.G. was partially supported by the European Research Council Advanced Grant FroM-PDE, by PRIN 2010-11 Grant "Geometric and analytic theory of Hamiltonian systems in finite and infinite dimensions" of Italian Ministry of Universities and Researches and by the FP7 IRSES grant RIMMP "Random and Integrable Models in Mathematical Physics". The work of B.D. was also partially supported by the Russian Federation Government Grant No. 2010-220-01-077.