Fix bug preventing prefetch data from being loaded if local data was provided

Previously if local data was provided, the prefetched data would never get loaded. To fix this I refactored *dataset.js* – there's now a function for processing raw data **and** a function for merging processed data into the existing `adjacencyList` and `itemHash`.

@timtrueman and @vskarich – as soon as I get a :+1: from one of you guys, I"ll merge this into `integration-0.8.1`, do some last minute administrative work, and then release v0.8.1 (!).