support copy-free IOStream

[stevengj]

Right now, your convert(IOStream, msg) function makes a copy of the data into a MemIO stream. (PS. memio is now deprecated in Julia, you should now use IOBuffer. And the function should probably be IOBuffer(msg) rather than convert.)

However, in the longer run it would be nicer to declare a new type ZMQ.MsgStream <: IOStream in which MsgStream(msg) wraps around a ZMQMessage (or ZMQ.Msg or whatever you decide to call it after #12) and provides a (read-only?) stream interface to the underlying data without making a copy.

[JamesWrigley]

Do we really need IOStream support? The docstring says it's specifically to represent file descriptors, which I don't think we have an analogue for with zmq_msg_t. I think IOBuffer(msg) already does what we want since Message is an AbstractArray.

Relevant PR: #212.

[stevengj]

Yes, this is probably obsolete.