Add api for certificate revocation

Author: remotelyliving

Certificate.php

@@ -39,6 +39,14 @@ public function __construct($certificatePEM, self $issuerCertificate = null)
         $this->issuerCertificate = $issuerCertificate;
     }
 
+    /**
+     * @return string
+     */
+    public function __toString()
+    {
+        return $this->certificatePEM;
+    }
+
     /**
      * @return Certificate[]
      */