org-id fixups and minor changes

["Gustav Wikström" <gustav@whil.se>]

Hi!

I've pushed a couple of fixes and changes to master related to org-id.

First; a fix and a (major) speedup and method-change for how the
global caching works for ID's. The change in method is that providing
file's as arguments to org-id-update-id-locations no longer breaks the
existing id locations.

Second; I've added a customization so one can choose to cache the
id-locations as relative to the org-id-locations-file instead of as
absolute links. This helps a lot when running a system mirrored on
multiple machines where the absolute paths to ones documents might
differ, but where it's all the same relative to the
org-id-locations-file.

Third; I've added another ID generator method. The extremists might
say the new method is not unique enough but org-mode is a personal
system first and foremost, so I think there is merit to this new ID
method. It creates ID's based on current timestamp and doesn't try to
hide the timestamp from the user. One might call the ID's "natural"
since they are contain information in themselves. Certainly a good
thing for some. The new method will not be active unless explicitly
chosen by the user.

    As a side note, if using ID's together with attachments, try this
    new ID-generator out by setting org-id-method to ts (short for
    timestamp) and change the org-attach-id-to-path-function to
    something like "YYYY/MM/DDTHHMMSS" for more human readable
    attachment folders.

Org-id is next to undocumented in the manual so I didn't find a good
place to add this. A few lines are added to org-news though.

This is the first push by me without first doing an RFC. So,
naturally, if anything is out of order mail back and/or make changes
directly in the repo if needed. Tests passed anyways.

Kind regards
Gustav