Fix example jpg to png

According to https://github.com/learnboost/node-canvas#canvastobuffer , 
canvas.toBuffer will return buffer containing PNG data.

<img src="/captcha.jpg"> seems to work fine in most browsers, but it doesn't work in IE8 - IE10, and even in IE11.

IE screenshots (link might expire):
http://www.browserstack.com/screenshots/e39d4e6998280987116732a8346bf68cb5a4e465