lintian-junit-report: Use REXML to generate XML output

Parsing lintian output in two runs in order to collect summary of
results had shortcomings - extending the code further to support skipped
tests would require replicating conditional logic in two separate
places. One of the possibilities to fix this would be to keep all but
header output in memory, then print the header and dump the collected
output. However using REXML::Document seemed a better idea resulting in
more readable code. And in fact the change makes the code 5 lines
shorter! ;)

I use REXML::Formatters::Transitive for output, as it preserves exact
text format, even though this makes XML look a bit unusual.
lintian-junit-report: Support for marking tests skipped

This change brings a possibility to mark selected lintian checks as
skipped (in terms of JUnit report). Skipped test cases are considered
less than passed by Jenkins and marked on test result trend, but the
default JUnit results publisher does not set job result to UNSTABLE when
seeing them. Thanks to that you can ignore selected results but still
have them available in the report. The behaviour is controlled by two
new command line options. Example:

lintian-junit-report -w --mark-warnings-skipped \
	--mark-as-skipped=no-debian-copyright,no-copyright-file [...]

Last option takes a list of lintian tags to mark skipped.
Merge branch 'feature/marking-tests-as-skipped' of https://github.com/lkslawek/jenkins-debian-glue