Added sentry hook

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

I thought about having multiple possible names for every "special field", something along the lines of
```go
if serverName, ok := getAndDel(d, "server_name", "host", "hostname"); ok {
```
but that made the "special fields" even worse.
Alternatively we could allow people to provide their own mapping for them but it seems overkill.