Django gets aggregation support!

As for Changeset 9742 Django gets aggregation support. For those of you that don't know, I worked in this project as part of Google Summer of Code and I'm very glad to see this finally coming true!.

I also wanted to thank Russell Keith-Magee for mentoring me throughout the project and working in the code after GSoC was finished, Malcolm Tredinnick for giving amazing code reviews during the whole SoC and the many others that worked in getting this done.

I encourage you to check it out. The documentation is a great start! =)