Use original_item_url instead of image_url when <guid> tag is missing in feed

In my opinion original_item_url better describes feed item than the image_url. Images in feeds are much more uncommon than link to oryginal article.
My patch reworks _aggregation_add_item function to handle guid in the following order:
* guid if it exists
* original_item_url if it exists
* image_url if it exists
* title if it exists
* body in rest cases.