Implement Webhook Security

[mogilvie]

Slight change of #2 and #3 to fix signature header.

[mogilvie]

Alterations for consideration:

1. Added a check in webhook subscriber to recognise stripe test events and return a 200 code.
2. Added the invoice.finalized event and subscriber
3. Added Product model
4. Modified Plan and Product to suit latest config at stripe.
5. Added nickname and active to plans.
6. Created a Client wrapper and defined it as a service for injection. See issue Create a StripeFactory to be testable/mockable #4
7. Allow config to ignore the default subscriber to let the app bundle extend the subscriber with their own version.