Add changelog for 0.17.0

Update release date in changelog

Cut 0.16.1

Correct date of 0.16.0 release in CHANGELOG.md

Merge pull request prometheus#1130 from prometheus/rc

Cut release 0.16.0rc2

Fix date of 0.16.0rc1 release in CHANGELOG.md.

Merge pull request prometheus#933 from prometheus/cut-0.15.1

Cut Prometheus version 0.15.1.

Merge pull request prometheus#913 from prometheus/beorn7/release

Cut release 0.15.0

Merge pull request prometheus#906 from prometheus/beorn7/release

Cut 0.15.0rc3

Merge pull request prometheus#898 from prometheus/beorn7/release

Cut release 0.15.0rc2