SPARK-1813. Add a utility to SparkConf that makes using Kryo really easy

@mateiz 's earlier request was for <code>registerKryoClasses</code>, but <code>setKryoClasses</code> sounds fine too if you both agree.

It looks like using <code>Class[_]</code> does work, but using <code>Seq</code> makes it difficult to use the API from Java.