[VOICE-775] Add voice namespace and phone_number resource.

This commit adds a `Voice::PhoneNumber` resource, and a way to call
`client.voice.phone_numbers.create(...)` etc. This is achieved
using a `Delegator` class to forward method calls after the
`voice` namespace back to the client object to be handled by
`method_missing`.
[VOICE-775] Add VCR cassettes for voice actions.

I reckon these might be useful to others running the live specs,
especially given the fact that creating a phone number cannot be
repeated with the same data.