Adds a block configure syntax to the Rest client.

This makes instantiation of multiple clients with the same credentials easier.
For example:

    Twilio::REST::Client.configure do |config|
      config.account_sid = 'someSid'
      config.auth_token = 'someToken'
    end

    client = Twilio::REST::Client.new
    client.account_sid
    # => 'someSid'