Adding health check

[gfrankliu]

There are a few upstream health check modules but they don't seem to work with nginx-upstream-dynamic-servers since they were not designed to have the server/IP list updated. This would cause nginx crashing as discussed in #13

Recently I noticed a fork https://github.com/xiaokai-wang/nginx_upstream_check_module that supports changing server/IP list. This module is actually modified to work with https://github.com/weibocom/nginx-upsync-module that allows changing upstream IPs via etcd or consul.

Is it possible to make sure nginx-upstream-dynamic-servers work with that health check?