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
I like the concrete example. I feel like we can do a little more to signal to the reader that this is going to be a "core part" rather than a digression, but that is easily done during copy editing (i.e. don't worry about it yet.)