Implement boleto payment

[darkdks]

As a continuation of the task where the payment flow was abstracted, we can now easily implement payment by boleto without violating DRY principles. This will likely result in a 30-line file and a few tests.

[darkdks]

I started developing it; I just need to finish the step of identifying the type of boleto and run some tests.

https://github.com/buserbrasil/santander-python-sdk/tree/barcorde-payment

https://github.com/buserbrasil/santander-python-sdk/blob/barcorde-payment/santander_sdk/barcode_payment.py

[darkdks]

#35