Add Element Selector Option

This commit allows you to provider a selector to select which elements you want to be a part of the slider.

`$("#gallery").nivoSlider({ slideSelector: "img" });`

My use case was with using Ember and having to deal with `<script>` tags that are generated within the slider container.

``` html
<div id="gallery">
    <script id="metamorph-0-start"></script>
    <script id="metamorph-1-start"></script>
    <img src="someImage" />
    <img src="someImage2" />
    <script id="metamorph-1-end"></script>
    <script id="metamorph-0-end"></script>
</div>
```