Modularize controllers

This is a proof of concept PR that supports the scenario I asked about on the mailing list: https://groups.google.com/forum/#!topic/plataformatec-devise/1ZPdAPpjGAY

To reiterate:

We have a scenario in a project where we want to to use devise both in the main app, and inside an engine. Using the information found here: https://github.com/plataformatec/devise/wiki/How-To:-Use-devise-inside-a-mountable-engine this works reasonably well:

```ruby
Rails.application.routes.draw do
  devise_for :users
  mount Foo::Engine, at: "foo"
end

Foo::Engine.routes.draw do
  devise_for :foo_users, router_name: "foo"
end
```

However, all Devise controllers inherit from whatever is specified in the shared initializer, i.e. ::ApplicationController. 

This means code from the base ApplicationController is inherited by all the engine DeviseControllers. Code that does things that are not meant to be done in the engine, for instance.

```ruby
class ApplicationController < ActionController::Base
  # code only applicable to the application
end

class Foo::ApplicationController < ActionController::Base
  # code only applicable to the engine
end
```

By extracting the devise controller code into modules, it is possible to choose inclusion over inheritance. That is, we can now do the following:

```ruby
class SessionsController < Devise::SessionsController # Business as usual
end

class Foo::SessionsController < Foo::ApplicationController # Not inheriting from a devise controller
  include Devise::Controllers::Base
  include Devise::Controllers::Sessions
end
```

What do you think about this approach? @lucasmazza @josevalim