Add an option to a PULL/PUSH socket with ZeroMQ

Hello,
This Pull Request adds a `multi` option to `ZeroMQHandler` and `ZeroMQSubscriber` in order to allow several handlers to write on the same channel. This option is really useful, as several independent processes can log on the same channel, and a single subscriber can listen to all of them.

I could have done it in my application by subclassing the handler and the subscriber (as it has been done before: https://github.com/zacharyvoase/logbook-zmqpush), but I think that this is a feature that could benefit to a lot of people, as logging via ZMQ really takes its sense when used with several processes, not just two.

The code could be simplified a little bit if #70 was merged, but it's not a big deal.

I'm using this code since a week with 5-6 processes writing a lot of debug simultaneously and it works fine.