Added support for sandbox resource

[programmarchy]

This commit allows a trial user to use the sandbox phone number in the same manner one would use the PhoneNumber object:

var sandbox = client.getSandbox();
console.log('Phone number is: ' + sandbox.attrs.phoneNumber);
sandbox.setup(function() {
  sandbox.sendSms(...);
  sandbox.makeCall(...);
});

I've replicated the dial-list and sms-list examples in a sandbox example.

[truepattern]

would be a very useful feature when one want to test functionality before buying credits

[ukd1]

Closing as this library is deprecated in favor of the official library - https://github.com/twilio/twilio-node. Thanks for your efforts!