SASL PLAIN support and create topic API

You can now connect to clusters with PLAIN authentication:

let cluster = Cluster(brokers: [("localhost", 9092)],
                      clientId: "saslClient",
                      authentication: .plain(username: "franz", password: "password"))

You can also create new topics:

cluster.createTopic(name: "myTopic", partitions: 3, replicationFactor: 2)

1.0.1

This release has internal changes that require Swift 4.1

New consumer API

This release includes a new consumer API that makes it easier to start and stop listening to topics.
Here is an example using the new Consumer class:

let consumer = cluster.getConsumer(topics: ["foo", "bar"], groupId: "testGroup")
consumer.listen { message in
    let string = String(data: message.value, encoding: .utf8)!
    print(string)
}

The existing consumer classes, SimpleConsumer and HighLevelConsumer are still around but have been deprecated.

Swift 4 and Swift Package Manager

0.2.0

Add back file private where needed