Created an initializer for bootstrap-sass installed by bower.

Hi David, 
Your online book 'AngularJS with Ruby On Rails' is great!! I noticed that you are requiring bootstrap as css rather sass. I created an intializer which makes it possible to @import bootstrap.sass in application.css.scss.

Note that depending on the version of bootstrap-sass you installed using bower, the directory structure may be different thus the initializer needs to be adjusted.

If you think my change is valid and good for receta app, please pull it in.