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).
Provide `contained_by` lookups for the equivalent single valued fields
related to the range field types. This acts as the opposite direction to
rangefield__contains.
With thanks to schinckel for the idea and initial tests.
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
