Braintree

* Added unified message to ErrorResult
* Added ability to specify country using country_name, country_code_alpha2, country_code_alpha3, or country_code_numeric (see [ISO_3166-1](https://en.wikipedia.org/wiki/ISO_3166-1))
* Renamed Subscription retryCharge to retry_charge
* Added gateway_rejection_reason to Transaction and Verification
* Allow searching with date objects (in addition to datetime)
* When creating a Subscription, return failed transaction on the ErrorResult if the initial transaction is not successful

* Added unified TransparentRedirect url and confirm methods and deprecated old methods
* Added methods to CreditCard to allow searching on expiring and expired credit cards
* Allow credit card verification against a specified merchant account
* Added all method on Customer to retrieve all customers
* Added ability to update a customer, credit card, and billing address in one request
* Allow updating the payment method token on a subscription
* Added methods to navigate between a Transaction and its refund (in both directions)

* Use isinstance instead of type to cater to inheritance (thanks [danielgtaylor](https://github.com/danielgtaylor))

* Prevent race condition when pulling back collection results -- search results represent the state of the data at the time the query was run
* Rename ResourceCollection's approximate_size to maximum_size because items that no longer match the query will not be returned in the result set
* Correctly handle HTTP error 426 (Upgrade Required) -- the error code is returned when your client library version is no longer compatible with the gateway

* Added transaction advanced search
* Added ability to partially refund transactions
* Added ability to manually retry past-due subscriptions
* Added new transaction error codes
* Allow merchant account to be specified when creating transactions
* Allow creating a transaction with a vault customer and new credit card
* Allow existing billing address to be updated when updating credit card

* Updated is_success on transaction results to return false on declined transactions
* Search results now return a generator and will automatically paginate data
* Allow passing cardholder_name when creating transactions