Braintree

* Added improvements to unicode handling.

* Added a new transaction state, AuthorizationExpired.
* Enabled searching by authorization_expired_at.

* Added next_billing_date and transaction_id to subscription search
* Added address_country_name to customer search
* Added new error codes

* Added Customer search
* Added dynamic descriptors to Subscriptions and Transactions
* Added level 2 fields to Transactions:
* tax_amount
* tax_exempt
* purchase_order_number

* Added billing_address_id to allowed parameters for credit cards create and update
* Allow searching on subscriptions that are currently in a trial period using in_trial_period

* Added ability to perform multiple partial refunds on Transactions
* Deprecated Transaction refund_id in favor of refund_ids
* Added revert_subscription_on_proration_failure flag to Subscription update that specifies how a Subscription should react to a failed proration charge
* Deprecated Subscription next_bill_amount in favor of next_billing_period_amount
* Added pycurl dependency in place of M2Crypto for better cross-platform compatibility
* Added new fields to Subscription:
* balance
* paid_through_date
* next_billing_period_amount