Add PanicHandler to ClientConfig

As mentioned in https://github.com/Shopify/sarama/issues/49#issuecomment-28233788.

Still missing two "go" calls in broker.go. Not sure how to wrap it since there is no reference to the ClientConfig.

Sarama noob here. Does this make sense, @eapache, @burke?

You could use it like this:
```go
config.PanicHandler = func(err interface{}) {
  // log error, send it to errbit, whatever. possibly panic() it again afterwards.
}
```