travis: check for doxygen warnings

This introduces a script that will check for doxygen warnings in newly committed or edited files.
The reasoning for cppcheck (and others) is that we don't want to break the build if the error isn't introduced by the tested PR. This is what `MR` does.
We only fail on `AC`. In this PR the script runs twice but doesn't actually discriminate between `MR` and `AC`  - so it would make sense to change it, that it acually does (or else just leave out the discrimination at all).