Basic Dwolla class

Includes static methods for Dwolla->Dwolla (send_money), Dwolla->Bank
Account (withdraw_from_Dwolla), Bank Account -> Dwolla
(deposit_to_Dwolla), as well as checking the account balance for a
user, getting the data on a specific transaction (via the txn ID), and
toggling auto_withdrawal on a user’s account.  Does not currently do
anything good as far as a) getting initial user Dwolla setup done,
either account registration or oAuth tokening, or b) automatic
notification of transaction status changes (via web hook).