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))

 Align the parameters of a method call if they span more than one line.
If you break up an argument list, keep the arguments on their own lines and closing parenthesis on its own line from [bp](https://github.com/fs/guides/tree/master/best-practices)
```ruby
fill_form(
  :user,
  full_name: 'username',
  email: email
  ...
)
```

or

```ruby
fill_form :user
          full_name: 'username'
          email: email
          ....
```