Needs can be created

This adds the ability to create a need by making a post request to the `/needs` endpoint in the API, passing in the attributes for the need as JSON. If the need is persisted, a `HTTP 201` status is returned, along with attributes for the newly-created need. If the need is not valid, a `HTTP 422` status is returned, along with an array of error messages.

To support this, this pull request also adds an `Organisation` model, and a rake task to import a list of all organisations from `data/organisations.csv`. There is outstanding work in the product backlog to modify this to use the new [organisations api](https://www.gov.uk/api/organisations) from Whitehall.