Refactor CoinbaseScraper, use Sentiment package as an experiment

Instead of having three different classes for each of the things to get from coinbase, just pass in some options to a single scraper.

I looked up *actual* sentiment analyzers for node, and found [this one](https://github.com/thisandagain/sentiment). I think it works pretty well, and returns a neg/pos number just like we were doing before. I put it in there.

Make sure to `npm install` before testing this out.