Removing dependency on #ribbon

Hi OkGoDoIt,

nice office ribbon! 

In switchToTabByIndex function you have a line like "$('#ribbon .ribbon-tab').hide();" so basically if I decide to give my ribbon a different ID, this code will break. 

Also you get an id (	$.fn.ribbon = function(id) {) as parameter. This should not be necessary if call it with $("#someid").ribbon.

This pull request solves both issues.