Documented the expected API for QuerySet.datetimes
Updated and added tests for QuerySet.datetimes.
Added datetime-handling infrastructure in the ORM layers.
Added datetime-handling infrastructure for the GIS backends.
Implemented QuerySet.datetimes on SQLite.
Implemented QuerySet.datetimes on PostgreSQL.
Implemented QuerySet.datetimes on MySQL.
Implemented QuerySet.datetimes on Oracle.