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.