CompositeField for Django Models

This is an implementation of a CompositeField for Django. Composite fields can be used to group fields together and reuse their definitions.

Example:

class CoordField(CompositeField):
    x = models.FloatField()
    y = models.FloatField()

class Place(models.Model):
    name = models.CharField(max_length=10)
    coord = CoordField()

p = Place(name='Foo', coord_x=42, coord_y=0)
q = Place(name='Foo', coord=p.coord)
q.coord.y = 42

The content of composite fields are stored inside the model, so they do not have to fiddle with any internals of the Django models. For example p.coord returns a proxy object that maps the fields x and y to the model fields coord_x and coord_y. The proxy object also makes it possible to assign more than one property at once.

There are some more examples in the included tests.py.