standalone sprockets support

I suggest to use bootstrap-sass only with Sprockets. It useful for Rack-based application.
[Here](https://github.com/aderyabin/bootstrap-sass_sinatra_example) is sample application.
To avoid appending twice on Rails, shouldn't this be:

```ruby
if rails?
  register_rails_engine?
elsif sprockets?
  register_sprockets
end
```