Add ability to place named variables in language strings [J2.5]

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31587

This is the Joomla 2.5 PR of #880 & #1597
Merge remote-tracking branch 'remotes/upstream/2.5.x' into patch-3
Merge pull request #1599 from nonumber/patch-3

Add ability to place named variables in language strings [J2.5]