Fixed error about Undefined property JTableXXX::$newTags when saving items

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31339

When saving a items (articles, weblinks, etc), and php error reporting is on max, you'll get an error about Undefined property JTableSomething::$newTags in the php error logs.

This PR fixes the issue.
However, it solves this by initialising the newTags property on the JTable object. I am not sure this is the best way to do this, as this will initialise the property on ALL JTable objects.
Maybe good for someone with a coding-eagle-eye to look into this...

###Testing Instructions
Save an item (article, weblink, etc) and see if the error is no longer there.