Get rid of the legacy application

The legacy version was a bit messy so let's begin with a whole new
application on Rails 4.0
Generate a whole new application using Rails 4
Add the action_args gem

This will allow us to easily deal with strong parameters and also have
declarative arguments for controllers' methods.
Fix a deprecation warning for I18n errors
Add formtastic to easily build forms
Add the category resource

The category model aims at tagging the different articles. It has the
following attributes:

  * Title
  * Slug

The controller is simply a RESTful one without an index action since the
categories just act like tags ; we won't display a list of them. The
form is built through Formtastic.