Fixing articles and article model query. It was failing if there are mul...

Fixing articles and article model query. It was failing if there are multiple contacts of different languages assigned to the same user, resulting in duplicated content shown.

There is also a huge performance loss if there are many articles due to the GROUP BY statement. According to the discussion on Google Groups:
https://groups.google.com/forum/?fromgroups=#!topic/joomla-dev-general/sVFw9reqGGw