Use reserved and managed entry mixins for managed directories.

File and directory names are given from the root of where they are accessed.
So:
* (Container).managed_file?("META-INF/container.xml") #=> true
* (ManagedDir).managed_file?("META-INF/container.xml") #=> false
* (ManagedDir).managed_file?("container.xml") #=> true
Better initialization of managed entry support.

There is a now an init method for classes that mixin the module to call.
Fix creation of subclassed Container objects.

When the initial file has been created it was then not being reopened correctly.
Add extra tests for managed entries.

Test the creation of subclassed containers with required files and directories,
including validated content.