Add a script to update the popolo JSON schemas

This iterates over the schemas in the schemas/popolo directory and
updates them with the latest version from popoloproject.com.
Update popolo schemas

Update the popolo json schemas with the latest versions from
popoloproject.com.
fix elastic search date mapping and search issues

Elastic search isn't happy with saving blank dates so we put in some
place holder ones and mark them as such when indexing documents. We then
remove the place holders and markers when we pull the dates back out.

This also means that death_date:>$some_date etc works as expected when
using the search.

FIxes #63 and Fixes mysociety/popit#610