Added sentry hook

Added a hook to log errors to sentry, it depends on getsentry/raven-go.

This is usually the platform (language) the logs are coming from. For supported languages this changes the way the sentry server interface shows exceptions.
Since go is not yet supported this does not matter but, if they add support for it they will expect ```go``` and not ```go logrus```.
As for the application name there are other specific fields that can be used in sentry but I wanted to keep the "special fields" to a minimum (logger and server_name are a necessary evil which ensures that they are not filled with arbitrary values by the raven client)