Add EntityTimestampsTrait to handle dates

Refactorize timestamps() method to avoid re-writing it on each entity

src/Wallabag/CoreBundle/Helper/EntityTimestampsTrait.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Wallabag\CoreBundle\Helper;
+
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * Trait to handle created & updated date of an Entity.
+ */
+trait EntityTimestampsTrait
+{
+    /**
+     * @ORM\PrePersist
+     * @ORM\PreUpdate
+     */
+    public function timestamps()
+    {
+        if (null === $this->createdAt) {
+            $this->createdAt = new \DateTime();
+        }
+
+        $this->updatedAt = new \DateTime();
+    }
+}