Nginx accepted and handled

According to http://wiki.nginx.org/HttpStubStatusModule#stub_status these 2 `accepts` and `handled` values are *connections* counters. So I changed the plugin to use the existing `connection` DS.

To sum up, we have:

 * `nginx/nginx_connections-{active,reading,waiting,writing}`: connections gauges
 * `nginx/nginx_requests`: requests counter
 * `nginx/connections-{accepted,handled}`: connections counters

(I also rebased the patch from #227 on current master)

@octo & @patrickshan, what do you think ?