Adds a block configure syntax

This makes instantiation of multiple clients, request validators and capability tokens with the same credentials easier.
For example:

    Twilio.configure do |config|
      config.account_sid = 'someSid'
      config.auth_token = 'someToken'
    end

    client = Twilio::REST::Client.new
    client.account_sid
    # => 'someSid'

    capability = Twilio::Util::Capability.new
    capability.instance_variable_get('@account_sid')
    # => 'someSid'

    validator = Twilio::Util::RequestValidator.new
    validator.instance_variable_get('@auth_token')
    # => 'someToken'

[edit] Updated the usage from `Twilio::REST::Client.configure` to `Twilio.configure`. Added other examples where it can be used.