Add VoIP support with additional channel

[dwightwatson]

This extends #90 - I thought I was pushing straight onto that PR but seems I did something wrong. This PR still includes @chimit's work.

• I added a separate ApnVoipMessage that just sets the push type out of the box. It extends ApnMessage so you can use it just the same, with less boilerplate.
• I split the tests so the two channels are tested independently. This also meant breaking out some stubs so they could be re-used.
• I did look to reduce some duplication between the channels but that can always happen later. The send method isn't that long and it's very clear what's going on.