added watt hour unit

Hi Josh,
I've added a Watt Hour unit in data/derived.yaml.

The underlying reason is that I am building some code to allow rails to convert units according to I18n rules, using the atom.property field as a key for the I18n translation (which then does a convert_to on the Unitwise object). Anyway - more of that in another pull request. It just so happens that Wh is a very useful unit if you are monitoring electicity generation or consumption.

Kind regards

Steve