Import newer version of sigmoid.

Make it so I can log a cluster-wide metric, if I do it explicitly.
`check_graphite` is a probe for Nagios.

This script do a comparison of two metrics and generate a nagios output
base on the value of --warn and --crit passed to it.
Add our proxies for carbon.

The goal of the proxies is to rewrite metrics to normalize them, and to
multiplex the metric.