Refactor module to put admin settings code in its own .inc file.  This allows Drupal to load settings code only when needed (i.e. only when viewing admin/store/settings/uc_tax_wa, not on every page and not every time a tax is looked up).

#602698, #601088 by jhodgdon: Bug fix (avoid having duplicate tax line items in orders), and updates for compatibility with Ubercart 6.x-2.0-rc7

Initial commit of Washington State Sales Tax for Ubercart module, which calculates sales tax in Washington State using the state Dept of Revenue lookup service, for Ubercart.