GitLab

- 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`.

Also renamed the keyword argument to 'file' to better reflect what it actually does.

Also:
- Add LICENSE.txt to MANIFEST.in
- Read version string in setup.py from __init__.py (without import)

Refactoring for better testability

See merge request !2

- marked tests requiring a running CDStar server
- added a fixture implementing a mock server
- moved package into an intermediate `src` directory
  (see https://hynek.me/articles/testing-packaging/)
- added `tox.ini` for cross-version testing

Fix 3.4 compat

See merge request !1