make get and set more Julian, and work with v2

[stevengj]

Would be nicer to have get and set defined regardless of the ZMQ version, and just throw an error if the functionality is not supported. (Presumably, in ZMQ v2, get(msg, ZMQ.MORE) could just return 0 always, whereas trying to set the MORE flag to true would throw an exception).

However, it would be more Julian to use ref and assign for these functions, and to return the appropriate types. i.e. you should be able to do msg[ZMQ.MORE] to get the ZMQ_MORE property (returning a Boolean), and msg[ZMQ.MORE] = true to set it. (ZMQ.MORE would be a singleton type, so Julia would know how to dispatch this).

Actually, once Julia issue #1974 is fixed (@JeffBezanson tells me he is working on this), one could do msg.more etcetera, which would be the ideal Julian syntax.

[JamesWrigley]

I'm gonna close this because we now support getproperty()/setproperty!() for all the Socket/Context/Message options, and we require ZMQ v4 so I don't think we need to worry about supporting multiple ZMQ versions.