Extend the postMessage API to enable getting the student log from the iframe

[sfentress]

We recently updated the SPARKS application to be embeddable in an iframe, and added the very beginning of a postMessage API to allow a host website to pass in their own circuit specification. An example of this can be seen at http://jsfiddle.net/SamFent/Wz35r/1/

We would like to start to extend this API so that we can have more communication between the iframe and the hosting website. As a first feature, we would like to be able to get the current student log.

This way if a teacher embeds Sparks on their own website, they can add a little code to check if the student did any work.