Adds an AWS_MSK_IAM authentication mechanism which is described here:
* https://github.com/aws/aws-msk-iam-auth#uriencode

To use the mechanism pass the following keyword arguments when
initializing a class:

```
security_protocol='SASL_SSL',
sasl_mechanism='AWS_MSK_IAM',
bootstrap_servers=[
    'b-1.cluster.x.y.kafka.region.amazonaws.com:9088',
    ...
],
```

The credentials and region will be pulled using `botocore.session.Session`.
Using the mechanism requires the `botocore` library which can be
installed with:

```sh
pip install botocore
```

**TODO:**

- [ ] Documentation
- [ ] Tests
- [ ] Refresh mechanism for temporary credentials?

The two tests in `test/test_msk.py` should ensure that the changes to
`kafka/msk.py` do not break the authentication payload.

The authentication payload was validated using a real AWS Kafka cluster
before adding tests with the hard-coded signatures.