Add value check for nodeText function (bugsnag#184)

* Add value check for nodeText function

This will make it easer to follow breadcrumbs for events that happened on submit buttons ect.

* Add password security check to nodeText function

src/bugsnag.js

@@ -521,7 +521,10 @@
 
   // extract text content from a element
   function nodeText(el) {
-    var text = el.textContent || el.innerText || "";
+    if (el.getAttribute('type') === 'password') {
+      return 'Password value can not be shown or it is empty';
+    }
+    var text = el.textContent || el.innerText || el.value || "";
     text = text.replace(/^\s+|\s+$/g, ""); // trim whitespace
     return truncate(text, 140);
   }