Added exception notifications

Exception notifications can be configured by setting the
EXCEPTIONS_FROM and EXCEPTIONS_TO environment variables.

* Added the 'exception_notification' gem
* Added initializer that calls the middleware
Gah! Remove .powenv
Make sure we send out email using sendmail
Test ENV on frontpage
Correctly setup email ENV variables for exception notifier
Merge pull request #1 from michiels/add-exception-notification

Added Exception Notifier that can be used with ENV variables EXCEPTIONS_FROM and EXCEPTIONS_TO