Check browser version, blacklisting ie6 end ie7

Add an alert message in case of outdated browser. 
Quite the same as #7597, but implementing a blacklist for ie6 and ie7

Here is a screenshot
![screenshot from 2014-11-18 17 43 28](https://cloud.githubusercontent.com/assets/715065/5091635/6fe32bf0-6f4a-11e4-839e-7d6f2090d9c1.png)

yes sure, I can move the message inside `layout/flash` 

I put it in a separate file becasue it's not properly a flash message. Anyway, I reckon moving in `layout/flash` may be a cleaner solution... As you prefer