Foundation.topbar fix

I made a little fix on Foundation topbar calling .off('fndtn.topbar') before the on() calls. This is useful in the case of javascript generated content, for example a Backbone view that writes the entire body element (including a topbar) after the execution of $(document).foundation(). Before the fix, consecutive executions of $(document).foundation() alternatively enabled and disabled the topbar events. Now, after every execution of $(document).foundation() the topbar works perfectly.