Fix up a couple doc comments.
Move interface checks into tests.
Remove the reference storage implementation for later.
Add a Close method to FileStorage.
Add a Copy method to Document (and Metadata).
Add AddMetadata and RemoveMetadata methods.
Do not automatically call meta.SetID().
Do not store the metadata if the file is not stored.
Do not require that MetadataStorage also implement DocStorage.
stor -> storage
Add a reference filestorage implementation.
MetadataSuite->FileMetadataSuite.
Move non-file documents into own package.
Expose the data underlying each FileMetadata.
Move some details out of filestorage.
Add more metadata methods.