SPARK-1098: Minor cleanup of ClassTag usage in Java API

Our usage of fake ClassTags in this manner is probably not healthy, but I'm not sure if there's a better solution available, so I just cleaned up and documented the current one.

Author: Aaron Davidson <aaron@databricks.com>

Closes #604 from aarondav/master and squashes the following commits:

b398e89 [Aaron Davidson] SPARK-1098: Minor cleanup of ClassTag usage in Java API