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
The table explanation makes sense. However, the values to the right of the colons in this example seem like they could be referencing some sort of first-class data type (perhaps other signatures), but I don't think they are by reading ahead. How does this work? Do these function like static or dynamic types? Do they constrain the kinds of values that can be put in a `Protocol` column etc.? If not, what do they do?