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

### Overview

The idea here is that we can namespace voice concerns within the API client so that we can do things like this:

```ruby
ZendeskAPI::Voice::PhoneNumber.create(client, {:number=>"+14848101172", :country_code=>"US", :toll_free=>false})
```

or

```
client.voice.phone_numbers.create({:number=>"+14843348060", :country_code=>"US", :toll_free=>"false"})
```

We're planning on using this gem to talk to our voice API endpoints from classic admin controller to retrieve voice information. This is the first step in that direction. 

### Tests
Two things: 

* I noticed running `rake spec:live` that we have a lot of fails in there. I don't know if this was related to VCR data being missing (I didn't look too closely TBH). 
* However, testing phone number creation is tricky as you can only create a given phone number once, and after that you can't provision it again without Twilio getting upset. So to avoid others having this issue I pushed the VCR fixtures into to repo despite the directory being listed in `.gitignore`. If this is not cool please let me know.

// cc @lukkry (will include Steven + Morten) after we're happy here.