Subscribe to `account.application.deauthorized` events

When a Stripe Connect application receives an
`account.application.deauthorized` event, the tokens have already been
decommissioned, so the event_retriever is pretty much guaranteed to
throw a `Stripe::AuthenticationError`, and no subsequent event
subscriptions fire.

Instead, if we receive a `Stripe::AuthenticationError` from a known
account and the type is `account.application.deauthorized`, we can be
confident that this was an authorized webhook from Stripe. Under this
scenario, construct a `Stripe::Event` from the passed params instead of
retrieving the Event from Stripe's API and fire the subscribed hooks
directly.

Fixes #15
Normalized params passed to Stripe::Event.construct_from

The Stripe api expects params to be passed into their StripeObject's
with symbolized keys, but the params that we pass through from a
accont.application.deauthorized webhook are a HashWithIndifferentAccess
(keys stored as strings always.
Merge pull request #32 from rmm5t/account-application-deauthorized

Subscribe to `account.application.deauthorized` events