cache control

In order to refresh the captcha, the example given create a sort of unique url using unused random parameter:

```
'{{ path('genemu_captcha_refresh') }}?' + Math.random()
```
It will be cleaner to set correct HTTP response headers rather than using such a hack.
Here is a small patch that should do the trick. Not sure it will work for every web browser.
It is an assumed copy-paste from http://stackoverflow.com/questions/49547/making-sure-a-web-page-is-not-cached-across-all-browsers

It also moves the cache control from client-side to server-side.