Add support for RASA NLU for on prem conversations

[ojacques]

Add support of rasa_nlu, for those concerns with keeping the conversations "on premises"

[mbettan]

I have a working code, but I had to workaround. I had two issues

1. RASA only supports HTTP today, so I had to configure ngrok or nginx proxy HTTPS to HTTP
   Enabling HTTPS instead of HTTP - API.AI RasaHQ/rasa#570
2. Endpoint APIs call were wrong from your original code, so after discussion with rasa_nlu on Slack, they changed their endpoint to be compatible, it has been released already

Then it was working but there is no fullfillment support on RASA so it means we have to implement them in hubot. More efforts.

I can export you the code if it helps?

[ojacques]

@mbettan - it would be nice! Maybe even a gist if nothing else.