Move all the interfaces into one file.
Add missing doc comments to the interfaces.
Factor out Doc, DocStorage, and MetadataDocStorage types.
Add fake storage implementations for testing.
Move the simple implementations into their own package.
Clean up the wrapper tests.
simple -> basic.
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.
Fix up a couple doc comments.
Move interface checks into tests.
Remove the reference storage implementation for later.
Add blank lines before doc comments.
Clarify a comment.
Move interface testing into global scope.