add a completionTimeout to kill long running requests

Adds a completion timeout to handle killing long running requests. The timeout defaults to false and must be explicitly defined in the configuration.
I don't think we include `method` and `uri` in any of the other error messages. While I agree this is nice information to have, I'd rather have it in a more general location like [here](https://github.com/groupon/gofer/blob/master/src/hub.coffee#L93-L99) and keep the message here simpler. That would also remove the need of passing through all options.