Serbian language update

inc/lang/sr/adminplugins.txt

inc/lang/sr/draft.txt

inc/lang/sr/lang.php

inc/lang/sr/subscr_digest.txt

inc/lang/sr/subscr_form.txt

inc/lang/sr/subscr_list.txt

inc/lang/sr/subscr_single.txt

lib/plugins/acl/lang/sr/lang.php

lib/plugins/config/lang/sr/lang.php

lib/plugins/plugin/lang/sr/lang.php

lib/plugins/popularity/lang/sr/lang.php

lib/plugins/revert/lang/sr/lang.php

lib/plugins/usermanager/lang/sr/lang.php