Introducing backend healthchecking plugin

[Piotr Sikora]

> I'll try to find some time today, can't promise anything though.

It took a little more time than expected, but I just pushed modified version 
of health-check module into my temporary repository:
http://github.com/PiotrSikora/healthcheck_nginx_upstreams/

It communicates with ngx_supervisord via its API, which means that servers 
are taken out of the rotation when healthcheck fails.

DISCLAIMER: Health-check module doesn't work for me at all (checks always 
time out and because of that servers are taken out of the rotation by 
ngx_supervisord), but if it works for you then this modified version should 
work as well.

Best regards,
Piotr Sikora < piotr.sikora at frickle.com >