* Fix get_url(cachebust=true)
The previous implementation looked for static files in the wrong place.
Look in static_path, output_path and content_path. If file can't be
found in any of them, print a warning to stderr and fall back to using
a timestamp.
Add a test to ensure it also works in practice, not just in theory.
* Implement get_file_hash
6708f7637c