Patch to delegate book page access to individual users

Here's a rather large patch against rc1 to extend the functionality to delegate access to individual users as well as nodes.

It contains the following:

• Users are added via textfield autocomplete
• the patch creates an extra db table to hold the uids - this is created in an update function, so it wont break existing installations
• In a 3-tiered site administration system (site admins, section managers, editors), some admins may only want to allow their section managers to delegate editors to roles (or vice versa) - the patch creates 2 new permissions to delegate to role/delegate to user. Users with 'administer book outlines' permission can delegate to either.
• Formerly people with the above permissions automatically got sitewide editing grants - however you may only want your section managers to have control over part of the site. So there is a hook to allow external modules to limit the section of the site the section manager can delegate editors to.

I can of course supply documentation etc - just waiting to see how the initial patch fares :)