REFACTOR make generator scaffold ready

Generator name changed, you can now use :

```
$ rails g darwinjs:assets <module_name>
```

For example :

```
$ rails g darwinjs:assets admin_area/contacts
      create  app/assets/javascripts/controllers/admin_area.coffee
      create  app/assets/javascripts/views/admin_area.coffee
      create  app/assets/javascripts/controllers/admin_area/contacts.coffee
      create  app/assets/javascripts/views/admin_area/contacts.coffee
```

If you want to generate CRUD modules, use a camelized name :

```
$ rails g darwinjs:assets AdminArea::Contact
      create  app/assets/javascripts/controllers/admin_area/contacts/index.coffee
      create  app/assets/javascripts/views/admin_area/contacts/index.coffee
      create  app/assets/javascripts/controllers/admin_area/contacts/edit.coffee
      create  app/assets/javascripts/views/admin_area/contacts/edit.coffee
      create  app/assets/javascripts/controllers/admin_area/contacts/show.coffee
      create  app/assets/javascripts/views/admin_area/contacts/show.coffee
      create  app/assets/javascripts/controllers/admin_area/contacts/new.coffee
      create  app/assets/javascripts/views/admin_area/contacts/new.coffee
      create  app/assets/javascripts/controllers/admin_area/contacts/form.coffee
      create  app/assets/javascripts/views/admin_area/contacts/form.coffee
```

This generator will be used as default for scaffolding :

```
$ rails g scaffold Foo name:string
      [...]
      invoke  assets
      invoke    darwinjs
      create      app/assets/javascripts/controllers/foos.coffee
      create      app/assets/javascripts/views/foos.coffee
      create      app/assets/javascripts/controllers/foos/index.coffee
      create      app/assets/javascripts/views/foos/index.coffee
      create      app/assets/javascripts/controllers/foos/edit.coffee
      create      app/assets/javascripts/views/foos/edit.coffee
      create      app/assets/javascripts/controllers/foos/show.coffee
      create      app/assets/javascripts/views/foos/show.coffee
      create      app/assets/javascripts/controllers/foos/new.coffee
      create      app/assets/javascripts/views/foos/new.coffee
      create      app/assets/javascripts/controllers/foos/form.coffee
      create      app/assets/javascripts/views/foos/form.coffee
      [...]
```

Please note you're still responsible to call module initialization
through `data-module` attribute in your view, since this generator does
not change erb generator.

If you don't want to use it when creating scaffold, put this in
`config/application.rb` :

```ruby
config.generators do |g|
  g.javascripts false
end
```

If you'd rather keep on using generic coffeescript generator, set:

```ruby
config.generators do |g|
  g.javascript_engine :coffee
end
```