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
What are some questions about the SOP that we could ask that are more easily answered with an Alloy model than by writing an SOP implementation from scratch? Why are these questions interesting?

I think if we can identify these questions and make them the "hook" for the chapter, the motivation for using a model-based approach (and Alloy specifically) will be even more convincing.