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
I also think it would be pretty powerful if we created some abstract diagram of a "mental model" of how HTTP works, and then show how a diagram of a single Alloy instance generated from the model compares to that original diagram :)