Added sandbox to settings

app/views/qbo/_settings.html.erb

@@ -57,6 +57,13 @@ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLI
       </td>
     </tr>
 
+    <tr>
+      <th><%=t(:label_sandbox)%></th>
+      <td>
+        <%= check_box_tag 'settings[sandbox]', @settings['sandbox'], @settings['sandbox'] %>
+      </td>
+    </tr>
+
      <tr>
         <th><%=t(:label_oauth_expires)%></th>
         <td><%= if Qbo.exists? then Qbo.first.oauth2_access_token_expires_at end %>