sort by mp

Output by sorted MP id order, so we can ensure that changes aren’t
making any material difference.
Generate MP stances by Person, not by Party

fixes a bunch of issues with people who change party, or have been
speaker, etc.
Add a Score class

Makes a lot of the other logic simpler.
Add a to_hash to Score
make_party_stances needs to deference the Score

The JSON class should really do this itself…
Fix make_mp_stances

Do in blocks of 20, and then merge them properly.