Adding support for crossdomain middleware

Notes: 
 - only one domain policy supported
 - secure value is not configured
imho this should rather be a complete single or multiline string (as there can be multiple entries of that or the like). and people will probably want to have secure="false" configurable as well. Imho a multiline text would be better here (this way people can specify multiple allow-access-from-domain statements for example).