Documented QuerySet.datetimes and updated QuerySet.dates.
Documented the new time lookups and updated the date lookups.
Updated and added tests for QuerySet.datetimes.
Updated tests of .dates().

Replaced .dates() by .datetimes() for DateTimeFields.
Replaced dates with datetimes in the expected output for DateFields.
Updated null_queries tests to avoid clashing with the __second lookup.
Added datetime-handling infrastructure in the ORM layers.
Implemented QuerySet.datetimes on PostgreSQL.
Implemented QuerySet.datetimes on SQLite.
Implemented QuerySet.datetimes on MySQL.
Implemented QuerySet.datetimes on Oracle.
Used QuerySet.datetimes in date-based generic views.
Used QuerySet.datetimes for the admin's date_hierarchy.
Documented backwards incompatibilities in the two previous commits.
Added support for parameters in SELECT clauses.
Made tzname a parameter of datetime_extract/trunc_sql.

This is required to work around a bug in Oracle.