django-joining 0.3

Arbitrary django model joining

Usage:

```python
from django.db import models
from django_joining import JoiningKey


class ModelA(models.Model):

foo = models.IntegerField()
bar = models.IntegerField()
baz = models.TextField()

model_b = JoiningKey('ModelB',
from_fields=['foo', 'bar'],
to_fields=['qux', 'quux'],
related_name='model_a_set'
)


class ModelB(models.Model):

qux = models.IntegerField()
quux = models.IntegerField()
quuux = models.TextField()



ModelA.objects.filter(model_b__quuux='test').select_related('model_b')

...

ModelB.objects.prefetch_related('model_a_set')

...

```