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 `0.3.x` nodes don't crush `0.2.x` nodes with uncompressed messages.

### Changelog:
* Bump the version in `node/constants.py`, `changelog`, and `README.md`.