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
A great question! We decided to introduce a concrete example mainly for the illustration purposes: (1) the generated Alloy scenarios in a previous version of the draft had objects with numeric indices like "Server0", "Client1", and some of the reviewers thought it was confusing, and (2) we felt that giving more concrete names to objects ("EmailServer" instead of "Server0") may make our examples seem more "real" to the readers, even though it's just really a naming issue.

Semantically, it doesn't really change the rest of the model, but we certainly don't want to give an impression that the modeler is required to provide a concrete example (after all, one of the things that make Alloy powerful is its ability to automatically generate scenarios from abstract models). What are your thoughts on the above two points? It's easy to revert back to the version without the concrete example, and this is still a point of contention between Daniel, Santiago, and I.