Added tests + bugfixes

Tests have been added to this release. Thanks to them I've found some bugs that I've successfuly bugfixed.

If you would like to test the plugin cd into the plugin directory, run composer install.

Then run phpunit into that folder after composer has finished installing. If you don't have phpunit you can easily add it with composer require phpunit/phpunit.

And then run php bin/phpunit inside the plugin directory.