Create multiple relation from owner

If we have a relation like this :

```
Book = db.define('Book', {name: String});
Chapter = db.define('Chapter', {name: {type: String}});

Book.hasMany(Chapter);
Chapter.belongsTo(Book);
```

To add multiple chapters in a book, we have to do something like this:

```
book.chapters.create({name: 'a'}, function() {
book.chapters.create({name: 'z'}, function() {
book.chapters.create({name: 'c'}, function() {

});
});
});
```

Or something like this:

```
book.save(function(e,b){

Chapter.create([{name:'a',bookId:b.id},{name:'b',bookId:b.id}],function(){});

});
```

So, it would be nice if we could directly store multiple child objects like this:

```
book.chapters.create([ {name: 'a'},{name: 'v'} ], function() {}
```

Look at the hasMany relation implementation in jugglingdb's source code, likely in lib/relation.js or similar. Find the create method for associations and see how it handles single objects. The task is to extend it to accept an array of objects. Check existing tests for relations to understand the expected behavior and add a test for the new array feature.

Scritto dal modello di indicizzazione a partire dal testo della issue.