Add Rails 4 support

* Add Rails 4 support. I test it on Sprockets, Rails 4 and Rails 3 apps. Everything is work. In Rails app, you just need `gem 'csso-rails'`, no conflicts with Sass settings (but maybe it is a Gemfile order question).
* Same class and file names.
* Cleanup README.md.
* `Csso.install(env)` API for standalone.
* Lazy `js_api` loading to safe time if you didn’t use `csso` (in development), but load gem.