Add support for custom form enctypes

Currently, the checkout_confirm.tpl hardcodes the payment form with an enctype="multipart/form-data".
This doesn't work for certain payment processors like Authorize.net. So I propose that enctype can be the default unless otherwise specified.

To get around this, I've created a session variable called "payment_form_enctype" and put a check in checkout_confirm.php and tpl, as well as a session->delete command for it in the payment.php library file.

But perhaps it might be better to do this check at a larger scale for more than just payment (as it is possible that other processors like future postal rates might require a different encoding as well). Perhaps a form accessor function like:

makeForm($action, $method, $enctype)