Fix for Mageworx_xsitemap incompatibility

When installing Turpentine on a Magento installation that uses mageworx_xsitemap, the usage of 'sitemap/cms_page' on line 175 on Mage::getResourceModel() of Helper/cron.php is fundamentally broken due to a rewrite, which dumps a fatal error on this line.

This can be reproduced by installing the mageworx_xsitemap module (our version came bundled with the Mageworx_Seosuite), navigating to the cache management page in the admin panel, and clicking Flush Magento Cache.

A fix for this issue would be to check if the module exists and if so, alter the supplied argument in Mage::getResourceModel();

    public function getAllUrls() {
        $urls = array();
        $origStore = Mage::app()->getStore();
        $visibility = array(
            Mage_Catalog_Model_Product_Visibility::VISIBILITY_BOTH,
            Mage_Catalog_Model_Product_Visibility::VISIBILITY_IN_CATALOG,
        );
        foreach( Mage::app()->getStores() as $storeId => $store ) {
            Mage::app()->setCurrentStore( $store );
            $baseUrl = $store->getBaseUrl( Mage_Core_Model_Store::URL_TYPE_LINK );
            $urls[] = $baseUrl;
            foreach( Mage::getModel( 'catalog/category' )
                        ->getCollection( $storeId ) as $cat ) {
                $urls[] = $cat->getUrl();
                foreach( $cat->getProductCollection( $storeId )
                            ->addUrlRewrite( $cat->getId() )
                            ->addAttributeToFilter( 'visibility', $visibility )
                                as $prod ) {
                    $urls[] = $prod->getProductUrl();
                }
            }
            $sitemap = (Mage::getConfig()->getNode('modules/MageWorx_XSitemap') !== FALSE) ?
                                                            'xsitemap/cms_page' : 'sitemap/cms_page';
            foreach( Mage::getResourceModel( $sitemap )
                        ->getCollection( $storeId ) as $item ) {
                $urls[] = $baseUrl . $item->getUrl();
            }
        }
        Mage::app()->setCurrentStore( $origStore );
        return array_unique( $urls );
    }

The above change successfully resolves the fatal error that is dumped upon clicking the cache clear button (after checking the debug log output I can confirm this generates the correct URL's).

Would love to see this merged so I don't have to do this every-time there is an upgrade for Turpentine!

I do apologise if I got something wrong with this pull request, first time doing one!