Hi Christian,
This is something that we've looked at in the past and continue to keep our eye on. But until WhatsApp releases an official public API, we can't move forward with an integration. WhatsApp actively circumvents third-party authentication and keeps it a moving target, so that any gateway we offered would be unstable.
We had hoped that Facebook's acquisition of WhatsApp meant a coming public API, but it was announced at the F8 conference in March of this year that there are no current plans to launch a public WhatsApp API. Another possible future avenue is that
messenger.com will provide an integration. We're watching that.