Add web components compatibility (polymer) - merging usablica#543 manually.

Author: afshinm

intro.js

@@ -1040,7 +1040,7 @@
 
     var parentElm = targetElement.element.parentNode;
     while (parentElm != null) {
-      if (parentElm.tagName.toLowerCase() === 'body') break;
+      if (!parentElm.tagName || parentElm.tagName.toLowerCase() === 'body') break;
 
       //fix The Stacking Contenxt problem.
       //More detail: https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context
@@ -1112,7 +1112,7 @@
   function _isFixed (element) {
     var p = element.parentNode;
 
-    if (p.nodeName === 'HTML') {
+    if (!p || p.nodeName === 'HTML') {
       return false;
     }
 
@@ -1175,7 +1175,7 @@
     overlayLayer.className = 'introjs-overlay';
 
     //check if the target element is body, we should calculate the size of overlay layer in a better way
-    if (targetElm.tagName.toLowerCase() === 'body') {
+    if (!targetElm.tagName || targetElm.tagName.toLowerCase() === 'body') {
       styleText += 'top: 0;bottom: 0; left: 0;right: 0;position: fixed;';
       overlayLayer.setAttribute('style', styleText);
     } else {