Context Options

Since libzmq 2.x only lets iothreads be specified as an argument to the context constructor while libzmq 3.x supports a general purpose API for setting context options, I took a tip from czmq and delayed actual context construction until it's required and followed the pattern already used in gozmq for socket options to support both 2.x and 3.x.  The result passes tests on both 2.2.0 and 3.2.2.