Add tests, minor consistency.

Changes:
- Adds the beginning of a direct-to-json-api test storage_json.js.
- Alters some API conventions for consistency
  (filenames starting with /user; text/x-pencilcode mime type)
- Removes the nginx_site.conf change for now.

I'd like to have us do the nginx_site change in a subsequent pull
request. The idea will be to let developers besides us exercise
this server as the devserver for a little while before we use it
as a prod server.