Support for different button formats

There are mainly two different ways to put the fb like button on a page:

1. an iframe: currently used by this module, doesn't need any javascript
2. a special element (HTML5 or XFBML): needs the fbconnect javascript, enables the use of javascript events

see https://developers.facebook.com/docs/reference/plugins/like/ .

While I don't think that this should become another module that inserts the fbconnect javascript (we can rely on the fbconnect module for that) it would still be nice to use the HTML5 version if the javascript is loaded anyway as it enables us to use advanced features like javascript events.