added multipart envelope support

[timdonegan]

More complicated zeromq patterns require the more complex "envelopes": http://zguide.zeromq.org/page:all#The-Extended-Reply-Envelope

[ggggggggg]

send_multipart + recv_multipart are clearly matched, while send vs recv_multipart are not. Also this is how pyzmq does it, which is a small plus. Regardless of the name for send, the functionality is useful.

[ggggggggg]

Actually it would probably be better to define send_multipart(socket::Socket, parts::AbstractArray) or send_multipart(socket::Socket, parts::AbstractArray{String}).