make #charge_vat_of and #snapshot private
refactor hook specs to use cassettes; move idempotent specs; apply_vat idempotent
use cassettes + fix bug
add tk
do not add VAT line item if VAT is zero
vat_service#calculate to return vat charge object; add vat rate to invoice
add vat amount and vat rate to invoice metadata
add tk
extend vat service to support all EU countries
updated specs
Merge branch 'master' into vat_country_rates

Conflicts:
	spec/cassettes/apply_vat_success.yml
	spec/cassettes/create_subscription_success.yml
Merge pull request #21 from piesync/vat_country_rates

support all EU countries as primary registered countries for VAT calculation
more robust stripe handling
invoice service that brings everything together
create subscription endpoint to return Stripe subscription
refactor hooks
default to json
remove inconsistent spec
mimic stripe error api
make currency mandatory and the same of the invoice