Origin:
- http://llvm.org/svn/llvm-project/clang-tools-extra/trunk/clang-tidy/tool/run-clang-tidy.py?p=294607
- https://github.com/llvm-mirror/clang-tools-extra/blob/c2e903ec98385b82e35bdb303e411854a2e8c032/clang-tidy/tool/run-clang-tidy.py

Modifications:
- the python version has been frozen to python2,
  as the script is not python3-compatible
- added -warnings-as-errors option
- the run-clang-tidy.py script has been modified
  to return a sensible exit code when running on Travis CI,
  i.e. it honors exit code of the underlying processes
  causing build failures on pull requests
