Generate namespaced modules

When the number of modules grows, some namespacing is required for sanity.

```
$ ./script/rails g onionjs:module one
      create  app/assets/modules/one/one_controller.js
      create  app/assets/modules/one/one_view.js
      create  app/assets/modules/one/one.mustache
      create  app/assets/modules/one/one.css.scss
$ ./script/rails g onionjs:module one/two
      create  app/assets/modules/one/two_controller.js
      create  app/assets/modules/one/two_view.js
      create  app/assets/modules/one/two.mustache
      create  app/assets/modules/one/two.css.scss
$ ./script/rails g onionjs:module one/two/three
      create  app/assets/modules/one/two/three_controller.js
      create  app/assets/modules/one/two/three_view.js
      create  app/assets/modules/one/two/three.mustache
      create  app/assets/modules/one/two/three.css.scss
$
```