Use DATE_ATOM instead of _atom_timestamp2w3dtf()

The module uses a custom function _atom_timestamp2w3dtf() when it could just use the PHP-provided DATE_ATOM constant with gmdate(). Core uses gmdate in it's RSS feeds so it doesn't have to mess with timezones. Since the constant was included in PHP 5.1.1 and above, we can also define it ourself for users of PHP 5.1.0 and below. This would clean up a small, but significant amount of code, and helps make the date formatting code much clearer.