Twipsy doesn't respect custom classes in a template

Created by: ansman

If you have a custom template:

<div class="twipsy foobar">
  <div class="twipsy-arrow" />
  <div class="twipsy-inner" />
</div>

The class foobar will never be shown because setContent lazily remove all classes except twipsy using the native className property.

Instead twipsy should remove only classes that it can itself (fade, in, top, bottom, left, right)