CDK-496: Added the type parameter to the Datasets API

I also went through and extended the use of the type parameter down to at least Avro files
and CSV files. In order to support Parquet and HBase, I think more work needs to be done
on the underlying stores. But if I missed something, I can get those in before the merge.
Correct, this test checks that Datasets#load throws an IllegalArgumentException on null. I should have cleaned up parts that aren't actually required any more from the test I copied it from.