Move a utility function into allmydata.util.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Refactoring to move ContainerItem and ContainerListing.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
'tahoe admin ls-container' work in progress.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>