Support sending transactional SMS messages

Adds support for sending transactional SMS messages

Version 5.3.0

• The identify method has been updated to allow the customer ID to be specified separately from the attributes, using the customer_id attribute. This allows a person to be updated by identifying them by e.g.: their email address. Thanks to trwalzer, jrbeck and jeremyw for the original changes that this is based on.

• It is no longer possible to set the customer_id attribute on a person. This is a side-effect of the changes to the identify method.

Version 5.2.0

The identify method will now automatically use the cio_id attribute as the customer ID when calling the track service. This allows a customer to be updated using identify to modify the id and email attributes.

Support for sending transactional push messages

Support for sending transactional push messages https://customer.io/docs/transactional-api/#transactional-push-notifications

v5.0.0

What's Changed

• Transactional Messages - Update amp_body and plaintext_body to use body_amp and body_plain, previous format is now deprecated #95