Organizations CRUD

Adds organizations crud operations according to the user story from PT:
https://www.pivotaltracker.com/story/show/77974308

Accessible on Heroku instance: 
http://sleepy-reef-7794.herokuapp.com/admin/organizations

Implements organizations crud with cucumber tests (similar to the ones for partners) and rspec tests for the model. 
Sly, we are using the 'shoulda-matchers' gem that turns many of these validations into one-liners. Have a look at e.g. the recently refactored partner_spec.rb: https://github.com/betasve/osra/blob/organizations/spec/models/partner_spec.rb. More info about the gem's usage is here: http://thoughtbot.github.io/shoulda-matchers/v2.7.0/.