Add function 'arv/goto-line'. Rebind s-g.

Idea stolen from Magnar Sveen's config. This versions integrates with
'relative-linum' and keeps 'linum-mode' enabled if it was before
calling the command.


git-svn-id: svn://localhost/emacslib/emacslib/trunk@516 a9598837-e506-0410-a967-fbf0066ee963