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.
These need to check that the class is not null and that it is a specific or generic class because reflect isn't supported in HBase yet. We should store the class as well, like you've added to the FS implementation.