hard-coding '/' instead of base_path() breaks woopra js cache

When Drupal is installed in a sub-directory (http://example.com/drupal/is/here), and when the "woopra_cache" setting is ON, the Woopra js file is not included properly, because it's written as:

<script src="/sites/default/files/woopra/woopra.js" type="text/javascript"></script> 

instead of

<script src="/drupal/is/here/sites/default/files/woopra/woopra.js" type="text/javascript"></script> 

The attached patch fixes this by using base_path() instead of assuming "/" as the start of the src value.

(Apparently 6.x-1.x-dev doesn't support the "woopra_cache" setting, so I'm expecting this will only be useful as a patch for folks not ready to switch to dev.)