captcha border

I think a captcha with border will be much better, so...

* Without border:

![captcha](https://f.cloud.github.com/assets/1768066/1000479/865af28c-0a4c-11e3-8071-db128cc8b0d5.jpeg)
* With border:

![captcha_2](https://f.cloud.github.com/assets/1768066/1000481/9314b7e2-0a4c-11e3-83b6-9648318d4b4e.jpeg)