Braintree

* Add Transaction `update_details`
* Support for Too Many Requests response codes
* Add SubMerchantAccount object with associate objects
* Allow more parameters to be sent on SubMerchantAccount create
* Add SubMerchantAccount update
* Handle validation errors for SubMerchantAccount create / update

* Add AccountUpdaterDailyReport webhook parsing

* Add Verificationcreate
* Add options to `submit_for_settlement` transaction flows
* Update https certificate bundle
* Support environment settings with strings

* Add better defaults to client token generation when using an access token by consolidating client token defaults into ClientTokenGateway
* Add PaymentMethodGatewayrevoke

* Add VenmoAccount
* Add support for Set Transaction Context supplementary data.

* Add transaction to subscription successfully charged webhook
* Add new ProcessorDoesNotSupportAuths error
* Add support for partial settlement transactions
* Add constants for dispute kind
* Preserve backtrace when not wrapping HTTP exceptions
* Add date_opened and date_won to dispute webhooks
* Add support for searching transactions from oauth app
* Support AMEX express checkout