Rails 4 fixes.

Updated to require Rails engine for Rails version >= 3.1. JS and CSS paths were being incorrectly generated as `/javascripts/bootstrap-3.0.0.min.js` and `/stylesheets/bootstrap-3.0.0.min.css` in Rails 4 projects in local environments (development/test), instead of `/assets/bootstrap-3.0.0.min.js` and `/assets/bootstrap-3.0.0.min.css`. Railtie is still required.

Updated README to reflect that :netdna is required to be passed as no default values are set in the method signatures for `twitter_bootstrap_javascript_include_tag` and `twitter_bootstrap_stylesheet_link_tag` as they are in the matching *_url methods.

I'd suggest bumping the minor version to force updates on existing projects.

Great project! Coupled with `jquery-rails-cdn` this cuts down a lot of time in the asset recompilation. Thanks!