Version 3.0.0

See `UPGRADE-3.0.md` for information on how to upgrade and `CHANGELOG.md` for
a complete list of changes.

Big Changes:

- PHP 5.6 or 7.0+ are now required
- PHP 7's error exceptions are actual handled by `DefaultConsumer`
- NativeSerializer signs messages by default, SigningSerializer was removed
- MessageExecutor and `HandlerResolver` were removed in favor of a single
  `MessageHandler` interface. See the upgrade guide for how this changes your
  code.
- `DefaultConsumer` is no longer `final`.
- There is now a new `AbstractConsumer` that provides everything but a `once`
  method.