Credit card validator added (Ticket #15)

Doctrine/Validator/Creditcard.php

+<?php
+class Doctrine_Validator_Creditcard {
+
+    /**
+     * @link http://www.owasp.org/index.php/OWASP_Validation_Regex_Repository
+     * @param Doctrine_Record $record
+     * @param string $key
+     * @param mixed $value
+     * @param string $args
+     * @return boolean
+     */
+    public function validate(Doctrine_Record $record, $key, $value, $args) {
+        return preg_match('#^((4\d{3})|(5[1-5]\d{2})|(6011)|(7\d{3}))-?\d{4}-?\d{4}-?\d{4}|3[4,7]\d{13}$#', $value);
+    }
+
+}
+