Users#show

## Users [/users]
### Fetch user information [GET]

+ Request

    + Header

            Accept: application/vnd.api+json;revision=1

    + Body

            { "id": "1" }


+ Response 200 (application/json)

        { "user": { "id": 34, "bio": null, "display_image": "https://gravatar.com/avatar/7fc04373cf0c63e376baec602fec8f45.jpg?size=150&default=http://localhost/assets/default-avatars/5.png", "display_name": "Joãozinho", "email": "person0.9132588986740293@example.com", "facebook_url": null, "linkedin_url": null, "other_url": null, "profile_type": "personal", "twitter_url": null, "created_at": "2014-07-01T14:30:30+00:00" } ] }

Closes #7.
I strongly believe that we should use [active_model_serializers](https://github.com/rails-api/active_model_serializers) instead of `rabl`.

It would be more simple to use, since we will use the same structure of one element in more than one place, for exemple:

```ruby

class UsersController < BaseController
  def show
    user = User.find(params[:id])
    render json: user
  end

  def create
    user = User.new(permited_params)
    if user.save
      render json: user, status: :created
    else
      respond_with user
    end
  end

  def update
    # same thing as create
  end
end
```

Also, we wouldn't need to add all the modules that is needed to do view rendering, so it would be faster. And using serializers, it is just ruby! :D