Added sentry hook

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

This'll block your entire application if network/sentry itself is slow, which can be truly awful and block a thread. Either set a very strict timeout here, or we could have a Sentry goroutine (created in the initializer) that reads off a buffered channel and sends to Sentry—and this code then drops the exception if the channel is full.

I don't know in what kind of Software you use this, but something that often comes up in our infrastructure (not with our Go applications as much though) is overloading the error pipes when the entire infrastructure is spewing exceptions, so we throttle exceptions heavily. If it's not a major concern for your own applications, I'm ok with shipping without that—but it should be able to deal with latent environments.