Implemented support for turbolinks, when it's present.

To enable it, just place it in the Gemfile of your application:

```ruby
gem 'jquery-turbolinks'
gem 'turbolinks'
```

Bugs that need fixing include: some tests fail

This does _not_ place a dependency on turbolinks, it has to be in the Gemfile!
It is not good idea to use `defined?` in template. Helper will be better approach here IMHO.