\section*{Acknowledgements}

M. G. acknowledges the hospitality extended during his visits at the Abdus Salam International Centre for Theoretical Physics where
this paper was prepared.

R. Kh. acknowledges financial support of the Georgian National Science Foundation (Grant No GNSF/STO7/4-197) and USA Civilian Research and Development Foundation (award No GEP2-2848-TB-06).


