~ Added btn-group to radio buttons of FOFFormFieldRadio

Hi Nicholas,

This PR adds the btn-group class to radio buttons of FOFFormFieldRadio so they're rendered just as the Joomla 3 core radio buttons. I tested with Joomla 3.2.2 and 2.5 and it works for both versions.

Do you foresee issues with this? I don't think people would like to render radio buttons in the classic style? Of course they could create their own FOFFormField if they really want to.

Cheers,
Jurian Even