Slt response header support

@fosdev For DISCUSSION ONLY. Please review.

Technically, it is ready to be merged. But I'm not sure about SLT response header name and format. 
We should, probably, be able to configure the name of it via crichton.yml. 
This could be:

```ruby
 @descriptors[:routes] ||= (descriptor_document[ROUTES] || {}).each_with_object({}) do |(transition_name, transition_descriptor), h|
  h[transition_name] = Route.new(transition_descriptor[CONTROLLER], transition_descriptor[ACTION])
end
```