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))

```
require rails_helper
```

Probably it would be better to require `rails_helper`  here instead of requiring it for each spec test file? Also after that, the `--require spec_helper` can be removed, since it's required in the `rails_helper` ?