Nrefocus

- Include docs in sdist

- Update documentation and examples

- Move documentation from GitHub to readthedocs.io
- Add universal wheel on PyPI
- Update tests on travis with new versions of NumPy

- Code cleanup

- Padding is now available in all methods (2)
- Added new convenient submodule `pad`
- Bugfix: autofocusing did not return the correct focusing distance.
This resulted in a slight offset in the refocusing distance for
the method `autofocus_stack` when `same_dist=True` was set.
- New test functions for `pad`