Allow reverse lookups for OneToOneField(s)

Suppose you write the following.

```python
class Foo (Model):
    pass

class Bar (Model):
    foo = OneToOneField(Foo, related_name="bar")
    baz = IntegerField()

class FooResource (ModelResource):
    class Meta:
        model = Foo
        export_order = fields = ( "bar__baz", )
```

Currently this does not work, but with a minor adjustment this will work, and will produce nice results for CSV export, etc.