undo including 0.8.2-rc1 changes in changes.rst
pre-releases should not have their own changelog

It's too much hassle to have to rename files or edit the changes.rst file manually, etc.
* updated release script so that:
   - pre-releases use the same changelog *file* (it can of course be modified) than the final release
   - it detects if a release with the same "short" version string has already been done and do not update the changelog in that case
* renamed current release changelog (again)