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))

@timurvafin it looks like current implementation of `site_prism` doesn't let us use url_helpers without pain, please correct me if I'm wrong. 

What I suggest:

```ruby
# login_page.rb
class LoginPage < PageObject
  def visit_page(user) # And so on for other page objects
    visit user_path(user)
  end
end

# sign_in_spec.rb
feature 'Sign in' do
  before(:each) do
    login_page.visit_page(user)
  end
end
```

If this doesn't look consistent enough then I suggest to stick to default `site_prism`s addressable-generated URLs.