Follow-on to #2069 - Decrease stack size of MPTT rebuild task.
This reduces the likelihood that we'll hit a limit with gargantuan galleries.
Add @todo comments to Model_Item::valid_slug() and Identity::load_user().
Change "self" --> "static".  This follows the "private" --> "protected" change,
and allows late binding.  We couldn't do this with PHP <5.3.
Change PHP version requirement in /installer/index.php to match /index.php.
Merge pull request #369 from shadlaws/followon2069_k3

Followon2069 k3