Ticket #14030 - Refactor Aggregate to use ExpressionNodes

This patch refactors Aggregate into ExpressionNode, allowing for total composability of the two.

My original announcement was in this email to the developers list, and more detail can be found there:

http://groups.google.com/group/django-developers/browse_thread/thread/3d3aba985cd8471f/98618f82b635806a