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 is going a little fast for me. Can I see what the `Call` signature looks like? (Even if you have to hand-wave away the type parameterization required to make it polymorphic.) Also, this use of the word 'polymorphic (to reference type parameterization rather than the potentially-more-familiar reference to dispatching methods based on dynamic type at runtime in OO languages) might be confusing to more junior readers.