Allow topicSeparator to be changed

[wbarnha]

I really like this library, my only issue is that we need pub/sub payloads to be formatted [Topic, 'b\0', Message]. Would you be willing to allow this functionality to change so that I could call something like ZmqPubConnection.topicSeparator = b'new_sep'?

[smira]

@wbarnha lgtm, would you mind squashing your commits?

[wbarnha]

Done, thanks for getting back to me quickly.

[coveralls]

Coverage increased (+0.2%) to 89.078% when pulling 7bd075e on wbarnha:topic_separator_attr into 678522b on smira:master.