added sayhttpd - ported say_httpd from NodeJS Hubot

Hi!

Thanks for your great p5-hubot. I have written an say httpd plugin for p5-hubot inspired from say_httpd from nodejs hubot.

This sayhttpd plugin accept JSON Post request to /hubot/say. The API is secured via an secure has.

Example curl request:

   curl -X POST -d '{"room": "#test-channel", "secret": "foobar", "message": "Hello from JSON" }' http://localhost:8080/hubot/say

The serect can be configured for hubot via env HUBOT_SAY_HTTP_SECRET