Add an upgrade path from Drupal 6 to Drupal 7

Since the Drupal 6 version of the module stores data in a separate table and the Drupal 7 version uses Field API there needs to be an upgrade path from the Drupal 6 version to the Drupal 7 version.

Note this issue is for the Drupal 7 version of the module and I will switch the version once a D7 one is available.