Use system_settings_form() to display admin/settings/ife form

Please consider using <a href="http://api.drupal.org/api/function/system_settings_form">system_settings_form()</a> to provide more straightforward UI for localization of IFE's "general error message".

When I added ife_general_message to $conf['i18n_variables'], nothing changed at <a href="http://example.com/admin/settings/ife">admin/settings/ife</a>. When I replaced return $form; to return system_settings_form($form); in ife_settings_form(), i18n started to tell me This is a multilingual variable., but I started having two "Save configuration" buttons.

Expected behavior would be having only one "Save configuration" button with i18n support, too.