Added: variable slide duration

[brospars]

I added the support of attribute data-duration in slides elements.
If not set the slide duration is the time set in slider parameters, if set then the slide duration is the one specified in the attribute.

Example :

<div class="slide" data-in="scrollTop" data-duration="3000">
            <!-- Slides Elements ... -->
</div>
<div class="slide" data-in="scrollTop"><!-- Default duration set in slider parameters -->
            <!-- Slides Elements ... -->
</div>

Not a big deal but it helped in a personnal project.