Kafka check - first pass

Uses the standard JMX format to pull in metrics from Kafka.

We're missing metrics by topic which is currently a limitation of the JMX configuration. I think we'll have to add something comparable to how we post-process cassandra metrics (https://github.com/DataDog/jmxfetch/blob/master/src/main/java/org/datadog/jmxfetch/Reporter.java#L94-L104) because the Kafka metrics have the topic name in them. I think we'd also want to limit to a list of topics to handle the case where a person has hundreds of topics.

Here's how a per-topic metric looks in JMX as a bean:

kafka.server":type="BrokerTopicMetrics",name="check_runs-FailedFetchRequestsPerSec"