Initial import of a DAC test application
travis: check for doxygen warnings

This introduces a script that will check for doxygen warnings in newly
committed or edited files.
travis: added doxygen check script to travis config
Merge pull request #2023 from PeterKietzmann/add_test_periph_dac

test/periph_dac: adds a test for DACs
Merge pull request #2104 from OlegHahm/travis_doccheck

travis: check for doxygen warnings