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/Model/TimesheetStatistic.php

@@ -9,7 +9,7 @@
 
 namespace App\Model;
 
-use \DateTime;
+use DateTime;
 
 /**
  * Timesheet statistics for one user.