Fixed #3566 -- Added support for aggregation to the ORM. See the documentation for details on usage.

Many thanks to:
 * Nicolas Lara, who worked on this feature during the 2008 Google Summer of Code.
 * Alex Gaynor for his help debugging and fixing a number of issues.
 * Justin Bronn for his help integrating with contrib.gis.
 * Karen Tracey for her help with cross-platform testing.
 * Ian Kelly for his help testing and fixing Oracle support.
 * Malcolm Tredinnick for his invaluable review notes.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9742 bcc190cf-cafb-0310-a4f2-bffc1f526a37