Init docs in NewDocStorage().
Include the filename in error messages.
Add a missing doc comment.
Add more detail to a doc comment.
Drop unnecessary comment.
Drop unnecessary parentheses.
Fix gocheck imports.
Add a Close method to FileStorage.
Add a Copy method to Doc (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