Support for multiple servers for VoiceClient

[Tr00d]

In certain scenarios, clients want to connect to multiple servers (EU, US, APAC) from the same application.
Currently, the request is sent to a single URL, NexmoApiUrl.

Implementation - TBC

We could use a fluent approach to specify a particular server before doing a request.

VoiceClient client = new VoiceClient(...);
VoiceClient usClient = client.WithServer(VoiceServer.US);
VoiceClient euClient = client.WithServer(VoiceServer.EU);
VoiceClient apacClient = client.WithServer(VoiceServer.APAC);