Instead of using DjangoJSONEncoder, use base_field's value_to_string().
Note this means the serialization of e.g. IntegerRangeField now has
strings for lower and upper, so use to_python when they came back in
(same behaviour as ArrayField, hopefully, from where I also got the
set_attributes_from_name function).
Backport of 86d9b10dc33cc115fee2ecab40a569354ac55d15 and
8a842148b6deaab021526e2689279cf5e232945f from master
Added support for PostgreSQL range types to contrib.postgres.
- 5 new model fields
- 4 new form fields
- New validators
- Uses psycopg2's range type implementation in python
