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.
You could still end up with a null if Object.class was passed in to match the existing behavior. In that case, you'll try and get the class from the schema SpecificData.get().getClass() may return null.