Add support for filtering on Django JSONField

[sjkingo]

Given a contrived model:

class A(models.Model):
    data = JSONField()

a = A(data={"name": "James"})
a.save()

This allows queries such as:

?data__name__icontains=James
?data__icontains=James

[charanjit-singh]

There are some problems importig psycopg2