Don't force the account_sid and auth_token to be passed when initializing a client incase they were set in the block configuration

twilio-ruby v3.13.1 added the block configure syntax:
https://github.com/twilio/twilio-ruby/commit/5e5485fbf76c5fcc4b8bf1d04c1e288a9104b9e9

I moved `Account` and `Message` out of the `Twilio::REST::Client` namespace to match the official gem's structure and take advantage of most of the native functionality.