Add WebhookController.error_notifier attribute

To allow applications to report errors to their error reporting tools.

Changes:

1. Added a "WebhookController.error_notifier" class attribute and the code to call it
2. Updated Responder to store exception objects so they can be forwarded
3. Updated responder.backtrace to responder.exception.backtrace
4. Updated WebhookController to actually use ResponderSerializer

Fixes #55