travis: check htmlreport via --errorlist output
travis: check all xml files cppcheck generated during tests of htmlreport via xmllint as suggested in 'createrelease'.
travis: install deps as suggested by travis manual (via before_install)
travis: move htmlreport checks into /htmlreport/checks.sh and make travis run it.
Merge pull request #182 from matthiaskrgr/travis

Travis check --errorlist output via htmlreport, check xml output via xmllint