Context options: IOThreads and MaxSockets.

This commit adds IOThreads() and SetIOThreads(int) methods for Context and,
for libzmq 3.x, MaxSockets() and SetMaxSockets(int).

Errors that would have been returned from NewContext() will now be
delayed and returned from the first call that really requires a ZeroMQ
context to be created.  This is to support SetIOThreads in 2.x.