Merge branch 'release/0.6.0' into main

Release v0.5.1

- Fix issue with `setup.py` that prevented installation on some systems
  (#27).

Release v0.5.0

- Add the `emd_samples()` function (PR #26).
- Clarify docstrings.
- Update documentation in README.
- Refactor tests.
- Explicitly support Python 3.4 and 3.5.

Release v0.4.4

- Clarify docstrings and error messages

Release v0.4.3

- Use MANIFEST.in to include files for compilation and distribution.

Release v0.4.2

Re-add support for Python 3.3 and NumPy >=1.9.

Release v0.4.1

Python 3.2 and 3.3 are no longer officially supported.

Release v0.4.0

API Additions
- The minimum-cost flow associated with the EMD value is now accessible with
  `emd_with_flow`.

Release v0.3.0

- Add input validation.
- Fix `setup.py`.

Release v0.2.0

- Exposed extra mass penalty option (defaults to the maximum entry in the
  distance matrix)