Add a test to verify that calling viewable() before MPTT relationships will fail.
Change Item::children() and Item::descendants() to use the new
relationship approach and add sort orders on as appropriate.  Once you
add a sort order, you can't remove it, so I created an
unordered_children relationship that does not add the ordering by
default and fixed the one place that uses it.  For all the rest of
them, the pre-existing behavior is maintained (we use the album's sort
order by default when requesting the children and descendants
relationships).
Merge branch 'kohana_3' of github.com:gallery/gallery3 into kohana_3