Backmerging master 0.2.3

Back-merging the `0.2.3` release into `develop` so that branches stay in sync. No code change is actually needed on `develop`; see the merge commit for details. This doesn't make the `0.2.3` and `0.3.x` releases compatible, as they use different crypto schemes; it only ensures that `0.2.x` nodes don't crush when receiving an uncompressed message.

### Changelog
* Bump the version in `node/constants.py`, `changelog`, and `README.md`.

### Note
I am not sure what time to set in the changelog. I have followed the tradition of earlier logs but I don't know if it makes sense.