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)

Good

Could be a `layouts/messages` containing something like

```
= render "layouts/flash"
= render "layouts/outdated_browser" if unsupported_browser?
```