Make a fields.gridfs()

Make a gridfs field, like `fields.binary` but stored in MongoDB backend.

We could do:

```python
from mongodb_backend import fields as mongodb_fields

...

_columns = {
  'some_binary_field': mongodb_fields.gridfs('Label')
}
```

This should save the object id in the field of postgresql table and the content in gridfs.

Support for file versioning:

![image](https://f.cloud.github.com/assets/294235/476427/50718482-b7be-11e2-9188-e0a613c3dadd.png)

![image](https://f.cloud.github.com/assets/294235/476430/5ee53004-b7be-11e2-809a-6e565c1f91e2.png)