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 is a signature? Why am I declaring one? Is this like a variable, datatype, module, or other definition in a general-purpose programming language?