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 I've changed my mind, let's use rails url helpers. For sure we could use `posts_path(post)` in case we need URLs with params.