Initial version of the same-origin policy model in Alloy

This initial version contains Alloy models of:
* client-server model with HTTP 
* the same origin policy
* the cross-origin sharing 
* security property

To add for the future versions:
* model documentation
* JSONP
* PostMessage
This is the first mention I've seen of a calendar server / application. I think we can get away with just using generic resources here, as we did in the early sections (i.e. I don't think the idea of it being a calendar app is necessary for this example?)