Change README.txt to remove discussion of which Ubercart 2.x version to use (not needed in 3.x) and to add link to DOR website for determining location code.

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).

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.