Make sure it also works in scrolling parents

Until this patch the appear plugin did not fire any events in the following scenario:

<body>
<div>
  ....
  <div style="overflow-y: scroll">
    .....
    <div class="appear-marker"></div>
  </div>
  ....
</div>
</body>
Merge pull request #4 from fhackenberger/patch-1

Make sure it also works in scrolling parents