Add $docComment checks in JMS\TranslationBundle\Translation\Extractor\File\FormExtractor::parseItem. Fixes #106

For a value that's an instance of e.g. ```PHPParser_Node_Scalar_String```, [which value holds a string](https://github.com/nikic/PHP-Parser/blob/master/lib/PHPParser/Node/Scalar/String.php#L4), there's no ```getDocComment()``` method. This leads to ```$docComment``` not being set and a fatal error as explained in #106.