one-line fix to nitobe_utils allows Nitobe to be more easily subthemed

Subthemes in Drupal 6 make it much easier to maintain custom variations of a theme. I created a subtheme based on Nitobe today and everything worked except for the header images, which continued to be picked up from the Nitobe headers folder.

Changing the function used to discover the theme's path from drupal_get_path('theme', 'nitobe') to path_to_theme() fixes the issue and causes the images to be pulled from a headers folder located in the subtheme directory.

The attached patch takes it a hair further by first checking to see if path_to_theme()/headers exists, and if not falling back on the original call to drupal_get_path.