Copy auto generated _id to id when first saving 

Giving a doc that omits id

``` python
class Doc(Document):
    name = StringField()

doc = Doc(name="test")
doc.save()
```

The above document will be saved with {"name": "test", "id": null} in "_source" and generated "_id" only in toplevel hits.

There are 2 options
1. Copy generated "_id" in to "id"
2. Not adding "id" field to document _fields and when doc.id is called, serve doc._id
