Allow any date format on settings page

At the moment, you can only choose between several predefined date formats on the drupal settings page. But for example in Germany, none of the proposed formats is common, i.e. a normal date in Germany would be 24.12.2005 for December 24th.

The attached patch removes the dropdown fields with the predefined values and replaces them with simple text fields where the administrator can provide a date format that suits his needs. A link to the PHP manual's page explaining the allowed parameters is also displayed below the textfield.