[MERGE] Document code layout stuff

[Ian Clatworthy]

Martin Pool wrote:
> This changes the developer guide to be more specific about how things should be
> spaced/indented. 
> 
> There have been a few cases where reviewers have said "please do X" when pep8
> doesn't (or doesn't clearly) require X.  abentley, lifeless and spiv said on
> irc that they agree with these rules (if I've captured them accurately).

bb: approve

You may wish to wait before merging just in case anyone disagrees. But
it looks good to me and it's great to have this clarified.

Ian C.