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.
Actually, I think this should default to GenericData.Record when the property is not set and fail if the property is set but the class can't be found.