Make sure Kryo instantiator uses the thread context class loader

This fix ensures that Kryo can find classes located in the submitted jar or -libjars arguments.  You shouldn't need a fat jar anymore, just ensure that Scalding and all dependencies are installed on your cluster and then you can submit a jar containing only your job classes.

Interesting to compare this with https://github.com/apache/incubator-spark/blob/master/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala which uses the same approach.