A set of updates to the "_link" system:

 - Made the /comment links/ jump to the comments instead of to the top
   of the page.

 - Replaced the "Add comment"-button by a "add new comment"-link.
   (This should make UnConeD jump up and down of wild excitement.)

 - Moved all comment related logic from theme.inc and node.module to
   comment.inc and comment.module (where they belong).

 - Added some links to ease the site's usability/navigation.