Add Nexmo Gateway

[judy2k]

I'd like to add a gateway for the Nexmo Verify service. I work for Nexmo, and I'm happy to implement the gateway and maintain it. I've opened this ticket for discussion, because I didn't want to supply an unwanted PR.

Is this proposed gateway something you'd be prepared to merge?

[moggers87]

@judy2k I'd read @Bouke 's comments on #215

There's also the option to make the Nexmo gateway a separate package, so I'd keep that mind if you do go ahead and make a PR.