index() adds signature file and other additional fields

- _medadata_files() can now return paths in a single list in
  this order: files, entities, collections.
- post() now takes an "additional_fields" arg for fields not in
  the models.  Examples include parent_id, signature_file, etc.
- Functions for guessing things about the object from the path:
  model, object_id, parent_id.
- Index() chooses signature for collections and entities.
- Index() adds the additional fields when sending to post().
- Additional documentation, especially for index().
Merge branch '137-browse-ui' of https://github.com/densho/ddr-cmdln into 137-browse-ui