add sugar Context
move MessageTracker to sugar
move socket sugar to zmq.sugar
move version, poll to sugar
add missing methods to sugar socket
green/sugar compatibility fixes
Context._instance is back to a class method

allows inheritance
add zmq.error

standalone to avoid circular imports.  Really belongs in sugar.
adjust imports

all top-level imports come via sugar
minor py3 and import tweaks
add constants to sugar

sugar module handles the collections and version-dependent values
ctx-socket circular import avoidance is no longer necessary
add header to backend.py
fix miscategorized int64 sockopts
Merge pull request #294 from minrk/sugar

separate Python sugar methods from base libzmq API in core

clears the path for cffi backend and pypy compatibility

closes #290