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
If I wanted instead to have a single field that references a `Response`, which itself would be a signature that has  `receivedCookies` and `body`, could I do that?