PHPUnit loaded via composer

About 18 machines test this extension.
Every machine might have different PHPUnit-Version,
which is inhomogeneous.
To keep the tests homogeneous the PHPUnit-Minor-Version
is fixed via composer.
PHP-Code-Sniffer tests for WordPress coding standards

The plugin will be read by many WP developers.
Every developer has his own style but WordPress has a standard.
To fulfill the WP coding standard the code is testable against that.

- WordPress-VIP coding standards are used
WordPress Test-Libs will be stored in project

The test-libs of WordPress were stored in /tmp folder.
The test-libs are unreachable for the IDE
and are deleted after every restart.
Putting the test-libs in a local folder makes them permanent rechable.

- /vendor/wordpress-tests-lib will contain PHPUnit bootstrap for WordPress
Test suite admin added

Tests did run in only one global suite.
One suite can not cover the test depending on `is_admin()`.
The admin-Suite has been added to cover `is_admin()` dependencies.

- /tests/bootstrap-admin.php sets WP_ADMIN
  and tries to set $_SERVER['PHP_SELF'] correctly
- /.travis.yml covers admin tests
- /phpunit.xml global suite will exclude admin tests
  and admin-suite has its own directory
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