Adds IBU calculation method to Recipe

IBU uses BeerSmith's ibu_contrib attribute on the Hop object.  BeerSmith uses Tinseth formula, and
so we use Tinseth formula.
Refactors Recipe specs to use 'its' instead of it w/ subject
Adds SRM color calculation to Recipe
Adds pry as development dependency
Merge pull request #4 from cadwallion/ibu

IBU/Color