Update document.py

For me I had a EmbeddedDocument class
```
class EventRaw(mongodb.EmbeddedDocument):
    timestamp = mongodb.DateTimeField(default=datetime.now)
    data = mongodb.ListField()
```

Which a document class is using.

```

class Events(mongodb.Document):
    events = mongodb.ListField(mongodb.EmbeddedDocumentField(EventRaw))
    date = mongodb.DateTimeField(default=datetime.now)
    ....
```
Which was giving me an error.
```
File "/usr/local/lib/python2.7/dist-packages/mongoengine/document.py", line 65, in __init__
    super(EmbeddedDocument, self).__init__(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/base/document.py", line 47, in __init__
    if self._auto_id_field:
AttributeError: 'EventRaw' object has no attribute '_auto_id_field'
```
You might want the fix instead in the EmbeddedDocument class instead.

My temporary fix was to set the auto id of the EventRaw class.
```
class EventRaw(mongodb.EmbeddedDocument):
    timestamp = mongodb.DateTimeField(default=datetime.now)
    data = mongodb.ListField()

    _auto_id_field = False
```