Domain access support

Here is the patch.

It contains:
* It allows adding users to domains in the multiple add screen.
* It adds a new screen similar to the assign user groups screen for assigning domain.
* It honors the domain module's "user defaults" settings (admin/build/domain/roles).
* It gives the permissions: "assign any domains via userplus" & "assign own domains via userplus"

Also on my todo list for the patch is to have a setting to only display users of the current domain
when using the assign user roles & assign user groups pages.

- This is no longer on my todo list. The "assign own domains via userplus" permission pretty
much makes it unnecessary.