Allow Cross-origin resource sharing (CORS)

see http://en.wikipedia.org/wiki/Cross-origin_resource_sharing
and https://gist.github.com/richard-flosi/3789163
Change API:

changed:    /pressure -> /pressure/all
new:           /pressure/COM7
Adding IPv6 support to the API server
Adding a web interface to the web API.

Plan for the future: The API should be mounted into this web app, so that
you can either start the API alone or the Interface+API.