Warn about outdated browsers

This PR shows a Foundation alert if an old/outdated browser was detected.

For screens (@larz0) see http://www.browserstack.com/screenshots/3ac65841a4b0189d2b9b1671933b955002b291fc
According to the screens, the alert is shown for:
* IE <= 9
* Firefox <= 4
* Opera <= 11.6

Don't know exactly 'bout Chrome and Safari.