Add a "Differences from Muse" section to the docs

Since people ask this a lot, here is a rough list:
- Use the promise type instead of core.async chans since core.async doesn't have error semantics. Throwing an exception in a fetch in Muse will cause the channel returned from run! to never have a value, silently. Channels don't fit well here.
- Fix a few bugs, both from pull requests to Muse and a few I discovered after adding more tests.
- Avoid the differences between Clojure and ClojureScript.
- Change a few of the API functions for having a more Clojurey name: map instead of fmap, mapcat instead of flat-map. Also, delete symbolic aliases for such operations.
- Make identity explicit in the DataSource protocol instead of magically trying to guess it.
- Big refactoring.
- Add proper docs.