Change TwilioClient to allow specification of base url and proxy to use

The TwilioClient is hard coded to send all requests to https://api.twilio.com
I have create a emulator to ease desktop apps development and in order to change the url i would need to use reflection to dig into the underlying RestClient

I added 2 properties that operate on the underlying RestClient

BaseURL - I gave the property a public setter and a backing field - this calls the SetClientBaseURL funtion which sets the baseurl on the underlying RestClient

Proxy - Directly sets the proxy on the underlying RestClient