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.
Added datetime-handling infrastructure for the GIS backends.
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.