Changelog updates

There've been some requests (like #1626) for having a CHANGELOG. While we have one, it has only been used for one version. I personally think that it would be useful to maintain a changelog; for users, packages maintainers and developers that use the module.
For that reason I've done some modifications so that we can edit the changes for the version in development in the `@DEV@` section of the `CHANGELOG.md`, and the `realease.sh` script will set the correct version when run.
Additionally the changes will be added to the `versions.json` file and to the atom feed (using pandoc).