Improve PrestaShop detection

In the latest versions of PrestaShop the powered by link looks like this: <a class="_blank" href="http://www.prestashop.com" target="_blank">PrestaShop</a>, so the current RegExp fails!

This one is more generic: /Powered by <a\s+[^>]+>PrestaShop/
Merge pull request #322 from djfm/master

Improve PrestaShop detection