Allow CORS request for /domains route.

Don't know if it's the right syntax, but I rellay need "Access-Control-Allow-Origin" headers to be defined for this issue : https://github.com/YunoHost/admin_js/issues/60
<3

dynette.rb

@@ -98,6 +98,7 @@ get '/' do
 end
 
 get '/domains' do
+    headers['Access-Control-Allow-Origin'] = '*'
     DOMAINS.to_json
 end