added on_exit hook to api

i could not find this hook
it executes when the server get an INT or TERM signal

what i wanted to accomplish is to marshal out data when the server stops
is there a better way to do it?

here is how to use it:
```
require 'goliath'

class HelloWorld < Goliath::API
  def response(env)
    [200, {}, "hello world"]
  end

  def on_exit
    puts 'goodbye world'
  end
end
```