added is-duplicate class to all duplicate slides

added is-duplicate slides to all the duplicates to that they can be easily picked out in situations where developers need direct access to them.
Update idangerous.swiper.js

changed is-duplicate to `swiper-slide-duplicate` and added a var `slideDuplicateClass`
Update idangerous.swiper.js

changing the addClassToHtmlString to be more generic and not rely on ul.
Merge pull request #553 from GalCohen/master

added is-duplicate class to all duplicate slides