Remove ContentFile#modified method.

This is unused and leftover from a previous incarnation of the codebase.
Remove unused ContentFile.rename method.

This is a dangerous place to put the method since posts can't just be
renamed at will. Draft takes care of its own renaming in publish!.
Add tests covering Contentfile#title=.
Remove unused ContentFile#[] method.

This just delegates to #headers, which is used in the codebase.
Remove bitrotted ContentFile.all.

This method no longer works since .all needs a Site.
Always update timestamps on Post#save.

Other header values might have changed, so this must be done. Post
inherits from ContentFile so it'll use that method.
Automatically set publish/update times on #save calls.
Be stricter about FileDigest::Syntax.
Just-because inspect test.