change CHANGELOG to KeepAChangelog format

[basnijholt]

as @jbweston suggested here #232 (comment)

[basnijholt]

Because I have used https://github.com/github-changelog-generator/github-changelog-generator to generate the CHANGELOG, I think getting it in the KeepAChangelog format amounts to adding tags to closed issues and PRs.

[basnijholt]

I believe this is fixed.