Fixed #17260 -- Added time zone aware aggregation and lookups.

Thanks Carl Meyer for the review.

Squashed commit of the following:

commit 4f290bdb60b7d8534abf4ca901bd0844612dcbda
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 13 21:21:30 2013 +0100

    Used '0:00' instead of 'UTC' which doesn't always exist in Oracle.

    Thanks Ian Kelly for the suggestion.

commit 01b6366f3ce67d57a58ca8f25e5be77911748638
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 13 13:38:43 2013 +0100

    Made tzname a parameter of datetime_extract/trunc_sql.

    This is required to work around a bug in Oracle.

commit 924a144ef8a80ba4daeeafbe9efaa826566e9d02
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 13 14:47:44 2013 +0100

    Added support for parameters in SELECT clauses.

commit b4351d2890cd1090d3ff2d203fe148937324c935
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 22:30:22 2013 +0100

    Documented backwards incompatibilities in the two previous commits.

commit 91ef84713c81bd455f559dacf790e586d08cacb9
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 09:42:31 2013 +0100

    Used QuerySet.datetimes for the admin's date_hierarchy.

commit 0d0de288a5210fa106cd4350961eb2006535cc5c
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 09:29:38 2013 +0100

    Used QuerySet.datetimes in date-based generic views.

commit 9c0859ff7c0b00734afe7fc15609d43d83215072
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:43:25 2013 +0100

    Implemented QuerySet.datetimes on Oracle.

commit 68ab511a4ffbd2b811bf5da174d47e4dd90f28fc
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:43:14 2013 +0100

    Implemented QuerySet.datetimes on MySQL.

commit 22d52681d347a8cdf568dc31ed032cbc61d049ef
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:42:29 2013 +0100

    Implemented QuerySet.datetimes on SQLite.

commit f6800fd04c93722b45f9236976389e0b2fe436f5
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:43:03 2013 +0100

    Implemented QuerySet.datetimes on PostgreSQL.

commit 0c829c23f4cf4d6804cadcc93032dd4c26b8c65e
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:41:08 2013 +0100

    Added datetime-handling infrastructure in the ORM layers.

commit 104d82a7778cf3f0f5d03dfa53709c26df45daad
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Mon Feb 11 10:05:55 2013 +0100

    Updated null_queries tests to avoid clashing with the __second lookup.

commit c01bbb32358201b3ac8cb4291ef87b7612a2b8e6
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 23:07:41 2013 +0100

    Updated tests of .dates().

    Replaced .dates() by .datetimes() for DateTimeFields.
    Replaced dates with datetimes in the expected output for DateFields.

commit 50fb7a52462fecf0127b38e7f3df322aeb287c43
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 21:40:09 2013 +0100

    Updated and added tests for QuerySet.datetimes.

commit a8451a5004c437190e264667b1e6fb8acc3c1eeb
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 22:34:46 2013 +0100

    Documented the new time lookups and updated the date lookups.

commit 29413eab2bd1d5e004598900c0dadc0521bbf4d3
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Feb 10 16:15:49 2013 +0100

    Documented QuerySet.datetimes and updated QuerySet.dates.