simplify all() code using promises

Here's an example of how you can simplify the `all()` code using promises. Note that I changed the semantics slightly in one place - when you query a db with no docs, the promise is resolved with `{}` instead of `undefined`, which I think is less surprising.

Promises take a long time to grok, but there are a few things I can briefly tell you that helped me out with understanding them:

1) Inside of a promise's `then()`, you can do one of three things: return another Promise, return a non-Promise (which is the final object that gets resolved), or throw an error (which invokes the `catch()`, i.e. the `reject` handler).

2) `new Promise(function (resolve, reject) {})` is a fine pattern, but since PouchDB returns promises anyway, you can avoid doing it explicitly.

3) We use "chai as promises" and "mocha as promised," which is awesome because it means you never have to explicitly call `done()` inside of your tests. :)  Just return a promise and everything will be handled automatically.  I fixed up one of the tests for you to demonstrate. This has the added benefit of converting synchronous exceptions (e.g. TypeErrors) to async promise rejections that get reported by Mocha.
Notice I got rid of the `rows.length === 0` checking, since we can just return an empty object in those cases.