0.6.0 release

[alimanfoo]

This issue is to declare intention to make a 0.6.0 release based on current state of master (fd46c4c). The set of issues and PRs resolved in this release is under the 0.6.0 milestone.

Release checklist:

• double-check cythonized C sources all up to date
• double-check all tox runs locally
• git tag
• release to PyPI
• update conda-forge feedstock (Release v0.6.0 conda-forge/numcodecs-feedstock#28)

[alimanfoo]

Made a small mistake with the release notes, forgot to remove "in development" from the 0.6.0 release title before tagging. Have fixed this in master and pushed a v0.6.0.1 tag just to update stable on RTFD.

[alimanfoo]

Issue in tests has come up during conda-forge builds (#138). Will cut a 0.6.1 with the fix and try again with conda-forge.