Add fontawesome to wicket bootstrap extensions

Hi,

Here's a pull request to add fontawesome in the extensions module based in this open issue :
https://github.com/l0rdn1kk0n/wicket-bootstrap/issues/202

I've used his code from https://github.com/a1dutch/wicket-bootstrap.
I've added comments, javadoc and make it works!

It's my first PR so I hope everything is OK.

Regards,

Gabriel.