Add script for converting _id fields of inaccessible documents

As detailed in mysociety/popit#300 the migration tool creates records
with invalid _id fields. This script goes through the database and model
specified and converts the _id fields to a string. This makes the
records accessible again through the API.
Merge pull request #23 from mysociety/convert-object-ids

Add script for converting _id fields of inaccessible documents