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.
What's going on here? This is defaulted to Object.class and then passed when creating the reader. Should this class get the type from somewhere?