Model.all(fn) array.js support

Added array.js support to `Model.all`.

Allows for things like:

```js
User.all(function(err, users) {
  var names = users.map('names');
});
```

Only downside of this approach is that we change the signature (probably breaking things). This change makes `users` an array of JSON objects. Before it was an array of User objects. I think this is fine, because you can just do:

```js
var user = new User(json);
```

Otherwise, I could possibly add an adapter to array.js to support functions as getters, but even that would overwrite some of modella's existing API (ie. `find`).