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Massive rewrite of the CLI and lots of refactoring work.

See merge request !8

- Changed the concept of CLI context and workspaces (see README).
- Moved 'pycdstar3.client' module to 'pycdstar3.api'.
- Moved data classes from 'pycdstar3.api' to 'pycdstar3.model'.
- Introduced optional high-level API handle classes for remote resources.

Return FileDownload from CDStar.get_file

See merge request !6

py3.5 compatibility

See merge request !7

`pkgutil.iter_modules` yielding `namedtuple`s `ModuleInfo` is new
in 3.6. On 3.5 it yields simple tuples.

Since the API is documented as REST API, it would be nice to have access
to generic HTTP methods, e.g. to retrieve archive status generically,
rather than through the more specialised methods like `list_files`.