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
I feel like this entire security paragraph might better serve the explanation if it were moved to an earlier point in the chapter. I feel like much of the explanation up to now has been on implementation without first giving a good grounds for cause.