Introduce page objects & Upgrade RSpec & Remove Turnip

* Introduced [site_prism](https://github.com/natritmeyer/site_prism) for using page objects in feature specs
* Introduced [formulaic](https://github.com/thoughtbot/formulaic) to simplify form filling in feature specs
* RSpec upgraded to 3.0 version
* Turnip features replaced with RSpec scenario syntax

(raw commit history can be accessed [here](https://github.com/fs/rails-base/pull/151/commits))

@airatshigapov as I can see you suggest use rails url helpers here, and you included `Rails.application.routes.url_helpers` into base `PageObject` for that.

How are you going to deal with [parameterized URLs](https://github.com/natritmeyer/site_prism#parameterized-urls) usng this approach?