[imp] Update Message for Extensions

Please see this feature Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33569&start=650

And for a example this picture:
![Uploading extension_update_feature.jpg . . .]()

This PR is the same for Extensions Updtes like this for the Joomla Core: https://github.com/joomla-projects/joomla-cms/pull/69