allow kernel_name to be specified in url parameter

IPython/html/static/notebook/js/notebook.js

@@ -2275,9 +2275,13 define([
         // Create the session after the notebook is completely loaded to prevent
         // code execution upon loading, which is a security risk.
         if (this.session === null) {
-            var kernelspec = this.metadata.kernelspec || {};
-            var kernel_name = kernelspec.name;
-
+            var kernel_name;
+            if (this.metadata.kernelspec) {
+                var kernelspec = this.metadata.kernelspec || {};
+                kernel_name = kernelspec.name;
+            } else {
+                kernel_name = utils.get_url_param('kernel_name');
+            }
             this.start_session(kernel_name);
         }
         // load our checkpoint list