Add Customers.validateTaxInfo

Change default API version to v2, which adds support for CFDI 4.0

For instructions on how to migrate, visit:

https://docs.facturapi.io/docs/migrate

Support passing cancellation params

Fix send_by_email

Trying to fix send_by_email endpoint

add endpoint to validate tax ids

Load all classes

Fix get routes

Fixed bug in building URLs, which might have been affecting `createGl…

…obalInvoice`, and download invoice methods

- Add support for Retentions API