MEP for review element

Hi all,

Here's a MEP for the review element:

http://projectmallard.org/mep/mep0019

The review element would be a child element of revision that you can
use to record a review being done.

Together with revision status flags (
https://github.com/projectmallard/projectmallard.org/issues/33), and
with some improved querying/reporting in yelp-check, this would allow a
complete workflow around reviewing and revisioning.

Comments very welcome please.

--
Shaun