[feature request] use relative path in the file set by org-agenda-files

[zwz <zhangweize@gmail.com>]

The variable "org-agenda-files" can be a list of file names or a single file 
name. I prefer the latter, since it is much easier to maintain one ordinary 
file without modifying the .emacs.

So in my .emacs I just use:
(setq org-agenda-files (expand-file-name "agenda" org-directory))

However, it seems that only absolute path works in that agenda file, e.g.
---------------------------------
in /path/to/org-directory/agenda:
/path/to/org-directory/home.org
/path/to/org-directory/lab.org
---------------------------------
Here I hope that I can use "./home.org" and "./lab.org" instead, so that when 
I copy the "/path/to/org-directory/" to another place, I do not have to modify 
the agenda file at all.