[Balanced] Handle invalid credit card numbers that aren't 16 digits gracefully.
[Balanced] Fix typo.
[Balanced] Check if string or not.
Remove currency preference from Stripe Gateway

Use Spree::Config[:currency] instead
[stripe] Relying on incoming currency settings would be preferrable
Make always failing USA Epay spec as pending (for now)

This is truly a terrible gateway and nobody should use it.
Store both the customer id and payment profile id from Stripe

For #105