Update the bootstrap generator to work with Bootstrap 3

With this generator I was able to get the example application working with Bootstrap 3.1.

Here a screenshot

![example application with simpleform and twitter bootstrap](https://f.cloud.github.com/assets/47848/2486353/99613276-b125-11e3-8346-78033e8360d9.png)

Related with https://github.com/rafaelfranca/simple_form-bootstrap/pull/28