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
This model is getting complicated enough that I'd really like to have that initial "abstract diagram" of it so that I can refer back to it while seeing it implemented in Alloy. We're now up to 10+ components, which is far too many for me to keep in my brain :)