Support "sale" parameter to filter on products with valid special price.

Have added necessary code to support an additional "sale" parameter to filter on products with valid special price. E.g.:
`{{block type="Yoast_Filter/result" name="cms_product_listing" template="catalog/product/list.phtml" category="207" count="3" sortby="name" direction="DESC" pager="1" sale="1" }}`
will add an additional restriction to include only products with current special price from and to dates.