Fix pathname bug in ZipContainer::Dir.

Was using an instance method from a class method! Clearly a test missing too, so
that has been added.
Fixup docs to refer to containers rather than documents.
Fix the base class in the recursive methods.

Where a method has to recurse up the hierarchy within a container to get to its
root, make sure that the Container superclass is used as the parent - not just
File.

This was causing the Dir containers to fail.
Add an internal compatibility method Dir#entries.

Used to mimic the behaviour of Zip::File#entries for managed entries.
Add a compatibility method Dir#read.

This is so that the contents of directory entries can be read like those of zip
files.

Tests included to catch regressions.