DjangoObjectType attribute of type FileField should return its url

Current behavior is to return file name for `FileField` and `ImageField` type attributes of `graphene_django.types.DjangoObjectType`.

It seems completely unusable as we always need an actual url of the file or image on a client.

Thus the one is forced to implement something like the following for every `FileField`:
```python
class CompanyNode(types.DjangoObjectType):
logo = graphene.String()

class Meta:
model = models.Company
interfaces = (relay.Node,)

def resolve_logo(self, args, context, info):
return self.logo and self.logo.url
```