Document arguments
Fill in organisation_type for departments

We are only interested in whether an organisation is a ministerial department
or not at the moment.
Add an endpoint exposing organisations

This lets us build UI in search for scoping/filtering by organisation.
Remove unnecessary defensive code

We can safely assume that the organisation links will follow the expected
format, especially given that we intend to remove this code once Whitehall
supplies slugs.
Merge pull request #103 from alphagov/organisation_api

Organisation API