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 section so far has jumped pretty firmly into the implementation of a model; however, I never really felt like I understood why we were trying to model this in the first place, except for a vague desire to explore the problem space. I sort of said this earlier, but I think it would be good to provide a series of questions that we'd like to have answered as a first step; then explain why agile modelling is a good approach to answering these questions; and then finally _propose_ a basic model (maybe spend one paragraph and a diagram on it) before implementing that model in alloy. Right now we're identifying components of the model and implementing them at the same time, and I'm finding it hard to motivate why we are doing what we're doing.