And `LEVEL_NOT_FOUND` with `undefined`.

Passes the abstract test suite. Most other tests are commented out.

The `iterator.next()` method required a small change: previously we
had the callback signature `(err, entries, finished)` where
"finished" meant "reached the end of the data". Now we have a
promise for only `entries`. So an extra `next()` call (end-to-end)
is now needed to know that the end was reached.

Has a tiny performance cost, which I negated by optimizing the
passing of options from JS to C++. The end result is faster than
before. However, I didn't check if it blocks the event loop for
a significant amount of time. Benchmarking concurrent gets might
answer that, later.

Ref Level/community#118.

Depends on Level/abstract-level#59.

Too much hassle.

Instead of copying abstract-level docs and then tweaking it for
classic-level (on every release, which was a bit of a pain) the
README now only describes the differences.