Subscription Events - Helpers and Scripts

Changes

1. Add Subscription to Retrieve New Messages functionality
   1. ./lib
      1. Add RingCentralSdk::REST::Event
      2. Add RingCentralSdk::REST::MessagesRetriever
      3. Add RingCentralSdk::REST::Request::Simple
      4. Update RingCentralSdk::REST::Client - update send_request
      5. Update RingCentralSdk::REST::Request::Base - add headers
   2. ./scripts
      1. Add scripts/fax_download.rb demo script
      2. Add scripts/fax_download_new.rb demo script
      3. Add scripts/sms_to_chat.rb demo script to send inbound SMS to chat