Add method for accessing success state

Author: rastating

lib/wpxf/wordpress/xss.rb

@@ -145,4 +145,9 @@ def upload_shell(username, password)
 
     true
   end
+
+  # @return [Boolean] true if the XSS shell upload was successful.
+  def xss_shell_success
+    @success
+  end
 end