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 there is a place that this is well-described online in the Alloy documentation, we could simply reference that documentation instead of adding an appendix to the chapter.