Rename fully_described_by to described_by
Rename Message#method_name to Message#name
Minor refactor of Message comparison code
[#2] Simplify Message arguments, no longer a separate object
[#2] Rename MessageReturn to ReturnedResponse
[#2] Convert Message specifications hash to a single reponse object
[#2] Make Message reponses more polymorphic
Run specs in random order
[#2] Only one implementor need satisfy each message response