Declarations are another layer of abstraction between defining the
factories via the DSL and compiling the factories and their attributes.
Declarations know how to return their attribute(s), and running a
factory compiles the declarations before building all attributes on the
factory. This moves all the attribute compilation logic into one
centralized location on the Factory instance, which means traits (and
potentially other features down the road) can have individual attributes
overridden within child factories or through FactoryGirl.modify
Closes#205
Joe Ferris and Josh Clayton