Allow per-event timezone import

The following version of event_node_import_fields() allows the timezone to be set event by event. (The change just consists of adding a timezone entry to the array.)

/**
 * Implementation of hook_node_import_fields().
 */
function event_node_import_fields($type) {
  if (event_is_enabled($type)) {
    return array(
      'node_import_event_start' => t('Event: Complete start date or timestamp'),
      'node_import_event_end' => t('Event: Complete end date or timestamp'),
      'timezone' => t('Event: Timezone'),
      'start_year' => t('Event: Start year'),
      'start_month' => t('Event: Start month'),
      'start_day' => t('Event: Start day'),
      'start_hour' => t('Event: Start hour'),
      'start_minute' => t('Event: Start minute'),
      'end_year' => t('Event: End year'),
      'end_month' => t('Event: End month'),
      'end_day' => t('Event: End day'),
      'end_hour' => t('Event: End hour'),
      'end_minute' => t('Event: End minute'),
    );
  }
}