add restart_done hook

This allows me to be able to livereload on backend changes:

```
guard 'rails', daemon: true, pid_file: 'rack.pid', CLI: 'bundle exec rackup -p 3000 -s thin -P rack.pid' do
  watch('Gemfile.lock')
  watch('config.ru')
  watch('app.rb')
  callback(:restart_done) {  Guard.plugins.find {|p| p.class == LiveReload}.reactor.reload_browser(["whatever"]) }
end
```

I didn't include specs as I'm not sure how to test this. If you have any pointers it would be much appreciated.

Cheers,