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.
I think it would be fine right here because this is the first thing that most classes do. We should fail as soon as we realize that we can't load the class the user expects.