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
We could also motivate the next section by demonstrating how adding the SOP to our model disallows valid use cases -- I'm guessing there's a way to ask if a specific desirable scenario is possible in the model, and showing that the model says it is impossible.