Integrate updates for Bootstrap 3, augment README and auto-load `chosen-rails` dependency

Would you please consider this pull request to integrate changes made by @billguy to bring this into line with Bootstrap 3 and update gem dependency versions and by myself to augment the documentation to reference the required gem 'chosen-rails' and to automatically require that as a dependency. If the updated gem could be pushed to rubygems also, that would be very good of you. Many thanks in advance.