Allow non-slides in the .slide-group.

The way the slider works right now is that it counts the children of the `.slide-group` even if those children are not `.slide` this include elements without size like `<script>`. This is incompatible with emberjs that injects script tags into the DOM to keep track of bound elements. So if you do something like this:

    <div class="slider">
        <div class="slide-group">
            {{#each}}
                <div class="slide">{{this}}</div>
            {{/each}}
        </div>
    </div>

The DOM will look something like:

    <div class="slider">
        <div class="slide-group">
            <script id="metamorph-11-start" type="text/x-placeholder"></script><script id="metamorph-14-start" type="text/x-placeholder"></script>
            <div class="slide">foo</div>
            <script id="metamorph-14-end" type="text/x-placeholder"></script><script id="metamorph-11-end" type="text/x-placeholder"></script>
        </div>
    </div>

And the slider will think there's 3 slides.