Fixes #35 Multiple messages not supported with payloads

[charleswong28]

It's my first day on dialogflow so I am not sure I am doing it correctly. I've just tested it on Facebook and it's okay to send multiple payloads like this:

agent.add(new Text({'text': 'hello', 'platform': 'FACEBOOK'}));
agent.add(new Payload('FACEBOOK', {...}, { sendAsMessage:true });
agent.add(new Payload('FACEBOOK', {...}, { sendAsMessage:true });

Please don't merge until someone who is working on other integrations gets this PR tested. Your help with this PR is much appreciated.

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