Python-3 porting

1) For XML validation now used 'lxml' python module instead of outdated
and unsupported library by Roland Leuthe. It works _much_ faster and
actively maintains by community. Files in 'lib' directory untouched.
2) Fixed additional CRC calculation for messages containing arrays.
3) It works on Python-2.7 too (at least on Gentoo Linux ~amd64)