Lintian and build fixes

These patches fixes more than half of lintian warnings for the package and addresses all warnings at build time. Biggest changes:

*  Use dh_python2 instead of the deprecated pysupport to build. This adds python as a build-dependency.
*  Replace deprecated "dh_clean -k" with "dh_prep". raised debhelper dependency to >=7 for this. Have not increased the value in debian/compat although that is probably possible as well.
*  Source format is set to 3.0 (native). No further changes needed for that.