Check Doxygen as part of Travis
Actually check the number of warnings from Doxygen
Fix the doxygen issues
Only install Doxygen if we need it
Comment out the config options not supported by Travis
Only run doxygen twice if we found warnings
Add some doxygen, including a deliberately broken piece to test the new warnings via Travis
Fix the deliberate error, Travis should go green now
Merge pull request #471 from peternewman/e1.37-2

Check Doxygen as part of Travis