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
Why does a browser have documents? My browser doesn't have documents until I actually fetch some, and those disappear when I close it (mostly). Is that what this is supposed to signify?