SPARK-1255: Allow user to pass Serializer object instead of class name for shuffle.

This is more general than simply passing a string name and leaves more room for performance optimizations.

Note that this is technically an API breaking change in the following two ways:
1. The shuffle serializer specification in ShuffleDependency now require an object instead of a String (of the class name), but I suspect nobody else in this world has used this API other than me in GraphX and Shark.
2. Serializer's in Spark from now on are required to be serializable.