Add endpoints for organisation manipulation

We need endpoints to add nodes and types of nodes so that we can migrate
the current organisation structure we are using into Stagecraft.

Includes some fixtures about cheese, because they are things and I
lacked imagination
Update README.md for multiple apps

We now have multiple apps in Stagecraft and the README needs to be
adapted to convey this.

I've also split up the initialising instructions from the steps to add a
migration for a new model.