Catching fatal errors

I added the ability to be notified on fatal errors and/or warnings. It's based on the old sfErrorNotifierPlugin from Symfony 1

Ideally the handlers would be registered very early in the boot process to catch any errors with compilation, DI, missing files or caching etc, but because I'm using the DI Container I had to wait for it to be registered before attaching the handlers. The handlers are registered at the Kernel:Request Event (which is still quite early) so any fatal errors or warnings after that are emailed using the Notifier.