ui.accordion: Setting 'event' option after init does not rebind the event.

Setting the 'event' option of the accordion widget after init does not rebind the event since the event is bound on creation of the widget only.

I worked around this by adding the following lines to _setOption:

if ( key === 'event' ) {
    this.headers.unbind( this.options.event );
    this._create();
}

This is in the spirit of ui.tabs which calls its create method (_tabify) for almost all options set after init.

Recreating the accordion does not seem to do any harm, but I am not sure about this. Of course the unbinding should better be done in _create istelf before binding the event.