Add Stripe-aware currency converter

Requires currency JSON with conversion rates from an external provider
Adhere to Stripe API update 2014-06-13

Stripe renamed stripe_card.type to stripe_card.brand.

If you use the new API, you must recompile all your modules
including stripe.hrl to pick up the new record change.

All tests currently pass.
Bump to version 0.7.0

This includes:
  - currency converter with external exchange rate JSON file
  - added failure_code and failure_message to charge object
  - Stripe API Updates:
    - 2014-03-13: rename statement_descriptor to statement_description
    - 2014-06-13: rename card.type to card.brand
  - Fix type of Stripe prices to note a max limit of $999,999.99

Records have changed in this version, so you need to recompile any
modules importing stripe.hrl.

All tests pass.