Test files will be checked by PHPMD and PHPCS

WordPress has coding standards which are tested here.
The test scripts were not tested by phpmd and phpcs.
All test scripts will be included in the conding-standards-tests.

- /.travis.yml extended to test the files
Plugins: Show installation date

WordPress shows all plugins in the backend.
Sometimes you want to know when they were installed.
Installation date will be shown for all plugins.

- lemike-commons.php fetches the script
  when user is in the backend
- admin/plugins/install-date.php hooks in the last row
  and shows the creation time of the plugin-directory
- test-install-date.php covers the UnitTests for that
- test-sample.php no longer needed
- /.travis.yml only covers needed tests