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/Core.js

@@ -11,7 +11,7 @@ var html2canvas = {};
 html2canvas.logging = true;
 
 html2canvas.log = function (a) {    
-    if (html2canvas.logging) {    
+    if (html2canvas.logging && window.console && window.console.log) {
         window.console.log(a);
     }
 };    

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: []
     };  
-};
+};

src/Renderer.js

@@ -78,12 +78,17 @@ html2canvas.Renderer = function(parseQueue, opts){
         queueLen,
         a,
         storageLen,
-        renderItem;
+        renderItem,
+        fstyle;
       
         canvas.width = options.width || zStack.ctx.width;   
         canvas.height = options.height || zStack.ctx.height;
     
-          
+        fstyle = ctx.fillStyle;
+        ctx.fillStyle = "#fff";
+        ctx.fillRect(0, 0, canvas.width, canvas.height);
+        ctx.fillStyle = fstyle;
+
         for (i = 0, queueLen = queue.length; i < queueLen; i+=1){
             
             storageContext = queue.splice(0, 1)[0];