clear canvas before rendering to allow jpeg export

* normally when exporting jpeg any pixel that was transparent
   becomes black, make them white
 * check for window.console.log before use (avoid crash)
 * html2canvas.Parse: fix missing call to html2canvas.Util.Extend for the
   given options

src/Parse.js

@@ -47,6 +47,7 @@ html2canvas.Parse = function (element, images, opts) {
     children,
     childrenLen;
     
+    options = html2canvas.Util.Extend(opts, options);
 
     images = images || [];
     
@@ -1254,4 +1255,4 @@ html2canvas.zContext = function(zindex) {
         zindex: zindex,
         children: []
     };  
-};
+};