Add httpctrl event listener

*This event listener controls processes based on the response of HTTP server(GET request). It does stop them or start them based on if some string is contained in response body or not.*

**This is my use scenario:**
I have a video stream, which is controlled(started and stopped)  using supervisor. To connect it with web interface, I created this event listener, which allows me to control it automatically and on very sane way, thanks to supervisor and superlance :)

- I think this feature is basic and really useful, so I think it should be included in superlance.

Code, tests and docs included!