new API

This major-version update simplifies the Felice API significantly.
It is not a backwardly compatible change.

It removes retry, logging and metrics logic from the package entirely,
leaving it up to the caller to manage those for itself.
It also adds a context.Context argument to the HandleMessage method
signature. This is so that a long-running handler (for example one that's
spending time retrying) can know when to stop. See #52 for more details.

It also updates to use the latest version of Sarama, which provides consumer-group
support, so there's no need to use the external sarama-cluster package.

Issues #28, #28 and #35 are now redundant and will be closed.