Use constants for string/byte literals

Patch to support #107. 

This moves string/byte literals to constants. 

This is more reliable to maintain, and 2/3 compatibility can be accomplished with one call to `six.b` or equivalent on import of a module rather than an unnecessary function call during runtime whenever a literal is used.
I'm importing `nsq/event.py` here, whereas the `async.py` case was `nsq/nsq.py`. The share name's a little confusing, yeah.