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
As a newcomer to Alloy, at this point I'm not sure if a `Resource` is a magical Alloy-thing, or if you're naming a component of the model itself (that is, a "resource" on the web.)