Add refresh image button

Every captcha have a possibility to update the current picture by clicking on it our via special _refresh_ button.

I have added the refresh button to simple_captcha.
The one current requirement is that we need to have ```jquery_ujs``` on the page with captcha. I don't know how to include it via simple-captcha correctly (we need to include ```jquery_ujs``` if it hasn't included on this page already)