Docs on the CompositeField class.
Moved quote/unquote from contrib.admin.utils to utils.encoding.

These functions have also been generalized somewhat for use by composite
values.
Initial implementation of CompositeField.
Reimplemented composite IN lookups.

Introduces InConstraint, a new DB backend operation for more efficient
multi-column IN lookups. Now, instead of creating a huge DNF expression,
an IN expression is created on backends with support for tuple literals
and an EXISTS subquery is used otherwise.
Implemented lookups on CompositeField.