Fixed reverse many 2 many lookup giving FieldDoesNotExist error.

The current implementation only works one way (you can do a _rel_ search only from the model that has the related field defined). This way you can filter from both models.

```python

class A(models.Model):
    pass # your fields here

class B(models.Model):
    a = models.ManyToManyField(A)

```

So you can filter by 

`/app/a/?_rel_b__id__exact=1`

and

`/app/b/?_rel_a__id__exact=1`