Change "organisations" to "organisation_ids"

This allows us to use the "organisations" key to give
more detail about each attached organisation.
Use one-sided habtm for Need/Organisation references
Add new fields to the NeedPresenter
Disable wrap_parameters

As we're handling parameters in NeedsController
ourselves, we don't need the Rails magic that this
provides.
Swap out Capybara for Rack::Test with integration tests
Add integration test for creating needs