Add i18n support

First, I refactored the request_phase function since it was getting kinda messy :grinning:

After that, adding i18n support was as simple as adding `lang` to an array. As you can see, this way it's really easy to add support for new features.

As for the i18n feature, it allows you to do this:

```erb
<%= link_to 'Twitter', "/auth/twitter?lang=#{I18n.locale}" %>
```