added: set referer string when fetching addons. Credit spiff.
If addon was pulled as a dep, use the parent as referer.
If addon is an update, use the old version as referer.
if addon is installed manually, referer is blank.
git-svn-id: https://xbmc.svn.sourceforge.net/svnroot/xbmc/trunk@35026 568bbfeb-2a22-0410-94d2-cc84cf5bfa90