lintian-junit-report: Allow passing options to lintian

This patch adds support for passing options directly to lintian,
allowing things like

lintian-junit-report \
    --lintian-opt=-Xsource-copyright,copyright-file *.deb

for those who like to break Debian Policy on purpose :). To be sure that
arguments are passed correctly lintian process is now executed with
IO::popen.
lintian-junit-report: Accept multiple file arguments

Usage note promises possibility of passing multiple files to check,
which hasn't been actually supported until now.
Support POST_BUILD_HOOK at the end of the build process

E.g. useful if you want to run 'debc *.changes' as part of
the build process.
Make lintian-junit-report ruby 1.8 safe

The changes from commits 7a974f4f and 21b8527d don't work when
running on Ruby 1.8:

| /usr/bin/lintian-junit-report:85:in `popen': can't convert Array into String (TypeError)
| 	from /usr/bin/lintian-junit-report:85
lintian-junit-report: quote XML strings