The method DataPackage.get_data should probably be moved into the Resource class, and it should handle other data types (related to #11). Also, though, not all data types are necessarily tabular, so it might be good if we had two separate methods: one which loads in all the data at once, and one which iterates through the data (and throws an error if it's not tabular).

When the data is read in, the methods update_bytes and update_hash could optionally be called (I think they're currently not being used), and an error thrown if the data size or hash has changed.