Fix code formatting
Refactor handling of commercial_base option for USPS

* Move the test for commercial_base configuration into a private method
* Prepare code layout to handle commercial_plus configuration option
  with minimal changes.
Add commercial_plus configuration option for USPS
Fix commercial plus XML fixtures to be pretty printed
Refactor #commercial_base? and #commercial_plus? to use ivars
Extract commercial type into a private method
Extract default service into a private method
Refactor #default_service to use a hash lookup
Extract service into a private method
Extract service validity assertion into a private method
Refactor domestic rate field logic into a lookup table
Refactor international rate field logic into a lookup table
Refactor rate response handling to have less synthetic variables
Refactor commercial flag setting to use a lookup table
Refactor commercial_type to be set in #initialize

* Remove #commercial_base? and #commercial_type? which are no longer
  needed since the logic is only used in one place.
Support for more USPS containers.
Refactor the commercial_type to use the options arguments

* This allows the options passed into each method to be used, rather
  than only using the options in the constructor.
Adds usps domestic build rate request with container
Merge pull request #188 from Shopify/usps-containers

Support for more USPS containers.
Removes empty test definition