replace accidentally-deleted server.ru config

This was originally added in 8c7036660
We now serve activities from github repo, instead of CouchDB

All activities have been moved to new sparks-activities github repo,
https://github.com/concord-consortium/sparks-activities.

We can now proxy the path /sparks-activities/ to the github-served
gh-pages branch of that repo, available at
http://concord-consortium.github.io/sparks-activities/

README and server configuration files updated.

jquery.couch dependency removed.
Use postMessage API to support passing activity directly to iframe

Using postMessage, we can now embed SPARKS in an iframe and directly
pass in an activity object.

Embedding SPARKS with the url {sparks-location}/embeddable.html#postMessage
sets up the postMessage handler.

In the page script, you can then say

  iframe = document.getElementById('iframe');
  activity = {
    "_id": "activity",
    "circuit": ...
  ...
  };
  iframe.contentWindow.postMessage(JSON.stringify(activity), '*');