Add system log driver and use it by default

app/Core/Log/System.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Kanboard\Core\Log;
+
+/**
+ * Built-in PHP Logger
+ *
+ * @package Kanboard\Core\Log
+ * @author  Frédéric Guillot
+ */
+class System extends Base
+{
+    /**
+     * Logs with an arbitrary level.
+     *
+     * @param mixed  $level
+     * @param string $message
+     * @param array  $context
+     * @return null
+     */
+    public function log($level, $message, array $context = [])
+    {
+        error_log('['.$level.'] '.$this->interpolate($message, $context));
+    }
+}