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 think a practical example (as the ones used by [Mozilla](https://developer.mozilla.org/en-US/docs/Web/API/window.postMessage)) could really be useful here