[dev] Deny permissions

[Jason Rust]

> I don't think it is. I've committed the file - thanks!

Great.  I'm trying to come up with a patch so that it will be usable.
My thoughts on what needs to be added are this:
1. Add hook example to hooks.php
2. Add config variable to conf.php
3. Change Group::singleton() method to look up the config variable, and
if it's not null or empty call the factory method with the config param
and return that.
4. Change Categories:exists() to be case insensitive.

Does this sound right?

-Jason