Support API of ExceptionNotification gem 

I am not sure whether you want to support this. It was easy enough to implement in the consuming app as a simple wrapper to this gem. It might be confusing, because the ExceptionNotification gem automatically sends an email, while this would only send email if Riemann is appropriately configured. On the other hand, this makes this more of a plug-and-play alternative to the ExceptionNotification gem.

In particular, this would support calling Notifier.exception_notification(env, exception).deliver and
Notifier.background_exception_notification(exception).deliver methods.