Make ProductsXMLFeed class more DRY

[JPry]

While reviewing #118, I noticed that the ProductsXmlFeed class repeats this same portion of code multiple times:

return '<' . $property . '>' . $variable . '</' . $property . '>';

return '<' . $property . '><![CDATA[' . $variable . ']]></' . $property . '>';

I think there are two things could be improved here:

1. There should be a dedicated method to create XML. Possibly also a dedicated method to output CDATA content specifically.
2. Instead of concatenation, I would recommend switching to interpolation or sprintf() instead.