Add USPS commercial plus

This branch adds support for USPS Commercial Plus configuration.
@csaunders One other refactoring I thought of is to create a private method to return the commercial flag field based on the commercial type.

This would allow me to eliminate the testing of `#commercial_base?` and `#commercial_plus?` everywhere except in `#commercial_type`. I could then just set `@commercial_type` once in `#initialize` and be done with it.