Create API specifications for Backends/Clients

[pjbull]

We don't have a full definition of what we rely on each backend function to do. For example, this just came up in #52 when we were discussing if _download_file should create non-existent parent directories.

Some example q's to specify:

• Should functions create non-existent parent directories or not? For most blob store backends, _upload_file won't complain if a portion of the "path" does not exist. However, for _download_file, it's usually the case that pathlib won't create non-existent parent directories for you.
• Should we overwrite existing files on the cloud or locally? (_move_file, _upload_file, _download_file)
• Should _list_dir include itself?
• Which functions should have "cloud native" implementations? (E.g., _move_file should prefer to issue commands to the backend rather than streaming files through the local machine).
• Should _get_metadata have a specified return format or should we just let the relevant Path class handle picking out what a user should want from a dict?
• Should we formalize _is_file_or_dir as a required method with enumerated responses?