Commodity: add models to allow factorization of hard-coded commodities

Add the following models:

 * CommodityGroup: a container for commodities, such as 'vegetable'
 * Commodity: a commodity
 * Translation: polymorphic model that can be applied to any other model
   in order to provide I18n support (currently used for CommodityGroup
   and Commodity).
Commodity: add rake task to create commodities, commodity group and translations
commodities: add controller for json output
commodity: add has_and_belongs_to_many association between farms and commodities
spec: add tests for commodities, commodity_groups, translatables and farm associations