Expressions

I've created a few commits - one per section. I think I've simplified a few parts. I've also corrected some grammar and spelling.
A reference to the order by implementation? 

> On 20 Oct 2014, at 17:24, Anssi Kääriäinen <notifications@github.com> wrote:
> 
> In drafts/orm_expressions.rst:
> 
> >  API, the call graphs are easier to understand, aggregates are expression
> > -subclasses and annotation of expressions is fully supported by the ORM.
> > -Expressions can't be used directly in other calls yet, but it will be
> > -possible to extend the expressions work to allow 
> > -`.order_by(NullsLast(F('height') / F('weight')))` for example.
> > +subclasses and annotating a query with arbitrary expressions is fully supported
> > +by the ORM. Expressions can't be used directly in other calls yet, but it is
> > +possible, and has been demonstrated, that support for order_by can be achieved::
> > +
> > +  .order_by(NullsLast(F('height') / F('weight')))
> Hmmh, do we need reference here?
> 
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