Added check-stats.rb graphite plugin.

For all of you too lazy to read the file header, here's a copy of what this plugin does:

Checks metrics in graphite, averaged over a period of time. Multiple stats will be checked if * are used in the "target" query.

The fired sensu event will only be critical if a stat is above the critical threshold. Otherwise, the event will be warning, if a stat is above the warning threshold.