Ohai plugin

For users that have chef locally, they will also have ohai, a JSON-based inventory management-and-sort-of-metric-collection tool.  This plugin runs ohai, then filters and reformats its output to be tab-delineated for nad.  It is written in ruby, and sniffs out the ruby interpreter that ohai itself uses, so we're guaranteed to have ruby (and the json gem).

It is not a fast plugin; shelling out to ohai is expensive. Could do it better, but for a first pass, it's OK.