document popolo data model

Pages to document how to use the data model by using some common examples.

Also a tutorial on using the API and a page listing some of the client libraries available.
Including David Cameron's membership confuses me, as we're showing this in the context of what the Post referenced from the David Cameron record should be, so it seems circular. I think there's a mismatch here between showing what data you'll use to *create* these records, and what the records will look like, in context, once they've been connected. For most new users I suspect the first (ie what they'll personally need to create) is much more important than the latter.