Distinguish international and domestic USPS calculators because service codes returned from usps api are not mutually exclusive

Update USPS shipping rate names and fix a bug in checking max weight for countries.

Add USPS Standard Post and USPS Global Express Guaranteed (GXG)

Fix broken rspec

Fixes #196
Merge branch '2-2-stable' into 2-2-nemo-stable

Conflicts:
	Gemfile
	config/locales/en.yml
	spree_active_shipping.gemspec