improved code styles #158 (#172)

* use php-cs-fixer instead of phpcodesniffer
* updated scrutinizer config
* adjusted code yules to all files
* updated CONTRIBUTING guidelines

src/Event/UserPreferenceEvent.php

@@ -18,7 +18,7 @@ use Symfony\Component\EventDispatcher\Event;
  */
 class UserPreferenceEvent extends Event
 {
-    const CONFIGURE = 'app.user_preferences';
+    public const CONFIGURE = 'app.user_preferences';
 
     /**
      * @var User
@@ -64,7 +64,7 @@ class UserPreferenceEvent extends Event
         foreach ($this->preferences as $pref) {
             if ($pref->getName() == $preference->getName()) {
                 throw new \InvalidArgumentException(
-                    'Cannot add preference, a preference with the name "'.$preference->getName().'" is already existing'
+                    'Cannot add preference, one with the name "' . $preference->getName() . '" is already existing'
                 );
             }
         }