Add support for data-selector attribute

Hey guys,

thanks for the great plugin! We wanted to use it to provide a feature tour for our existing web-based software solutions. But we don't like to add extra-id's or classes just to be able to target elements with joyride. So I added support for a "data-selector" attribute which allows selecting the target element with a complex jQuery-selector instead of just ID or class.

Hope you like it :-)