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/Invoice/ServiceInvoice.php

@@ -50,6 +50,7 @@ class ServiceInvoice
                 return new $class();
             }
         }
+
         return null;
     }
 
@@ -72,6 +73,7 @@ class ServiceInvoice
                 return new $class();
             }
         }
+
         return null;
     }
 
@@ -97,6 +99,7 @@ class ServiceInvoice
                 return $action;
             }
         }
+
         return null;
     }
 }