Specify class name for calculator association in calculated_adjustment

Otherwise, top-level classes that include this module will have an association that looks for ::Calculator, rather than Spree::Calculator
pass country_id as data to jQuery.get

if your default url options happen to include ?locale=en then
the previous implementation would end up generating:
api/states?locale=en?country_id=49 instead of
api/states?locale=en&country_id=49

Fixes #3108