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
At this point, I was expecting to see how enforcing the SOP negated the counterexamples shown above (via another `Confedentiality` check.)