Fixed #14030 -- Allowed annotations to accept non-aggregates

This PR supersedes https://github.com/django/django/pull/2184 and fixes https://code.djangoproject.com/ticket/14030.

Annotate has learned to accept non-aggregates, and aggregates can now use F() objects and arithmetic, since they now derive from ExpressionNode:

```
Author.objects.annotate(combined_ages=Sum(F('age')+F('friends__age')))
Author.objects.annotate(double_age=F('age')*2)
```
 ML discussion: https://groups.google.com/forum/#!topic/django-developers/8vEAwSwJGMc

I'm going to close the previous PR which only implements the aggregates refactor. The additional commits for this PR allow non-aggregate annotation, and includes the beginnings of documentation.

There's still a little bit to be done, but I'm confident that both of these items can be completed in a fairly short timeline (within a week if required immediately):

- ~~Complete documentation~~
- ~~Implement deprecation path~~
  - ~~renamed aggregates to annotations attribute on sql/query.py~~
  - ~~sql/aggregates should no longer be used~~

I'm fairly sure the code is solid and ready for review. I admit, it's a big change. But it does increase the utility of aggregates and annotations significantly, and allows users to implement their own aggregates and expressions very succinctly.