Message struct for Mouth
phone_number() :: String.t
phone_number_list() :: nil | phone_number | [phone_number]
t() :: %Mouth.Message{body: String.t, to: phone_number}
body(Mouth.Message.t, String.t) :: Mouth.Message.t
new_message(Enum.t) :: Mouth.Message.t
to(Mouth.Message.t, String.t) :: Mouth.Message.t