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

Here's a thread that indicates order is important, at least in the sense that the kryo instance serializing should have the same order as the instance deserializer:
https://groups.google.com/forum/#!topic/kryo-users/E-0_EVi-O1Q

String's hash code is deterministic, so it shouldn't really matter, but I think it doesn't hurt to be conservative.

The LinkedHashSet should preserve order, unless I'm missing something?