Enable Overlay plugin

git-svn-id: http://dev.piwik.org/svn/trunk@7760 59fd770c-687e-43c8-a1e3-f5a4ff64c105
Fix problem reported in http://forum.piwik.org/read.php?2,99424
Strict Notice: Only variables should be passed by reference in .../core/Menu/Abstract.php on line 61

git-svn-id: http://dev.piwik.org/svn/trunk@7761 59fd770c-687e-43c8-a1e3-f5a4ff64c105
Fix problem reported in http://forum.piwik.org/read.php?2,99414
Notice: Undefined index: login_logout_url in /mnt/weba/d2/34/51062934/htdocs/piwik/plugins/Login/Controller.php on line 499

git-svn-id: http://dev.piwik.org/svn/trunk@7762 59fd770c-687e-43c8-a1e3-f5a4ff64c105