Adding "-none-" as an option for donation form layouts

Adding "-none-" as an option for donation form layouts (form_layouts.module). This will allow `webform-form-NID.tpl.php` templates to be used on donation forms, as needed.

The request originates from IRC ticket #643.

**Note:** this pull request only provides the "-none-" option. It does *not* change the default from `one_column`, so it will not affect any existing donation forms.