use ROUTER/DEALER sockets for stdin

stdin requests are now directed to the frontend that made the execution
request that called raw_input.

allow_stdin flag added to execution_request to indicate whether stdin_requests are allowed.

StdInNotImplementedError exception class added to IPython.core.error, and is raised if raw_input is called from execute requests where allow_input is undefined.

closes gh-673

IPython/frontend/html/notebook/static/js/kernel.js

@@ -121,6 +121,7 @@ var IPython = (function (IPython) {
             silent : false,
             user_variables : [],
             user_expressions : {}
+            allow_stdin : false,
         };
         var msg = this.get_msg("execute_request", content);
         this.shell_channel.send(JSON.stringify(msg));