Add install_prerequisites.py, simplify pre-commit, install.py, presubmit_impl.py

- Improve presubmit_impl.py output. It now prints the command which generated
  the error.
- install_prerequisites.py installs all the needed executables very quickly,
  much more than running the commands sequentially.
- install.py now calls install_prerequisities.py.
- samples.travis.yml now calls install_prerequisities.py.
- Removed redundant checks in pre-commit to speed it up.

The likelyhood of this commit to work on first try is near 0.