allow XSUB/XPUB to send/recv messages unrelated to sub/unsub (LIBZMQ-490)

zmq::xpub_t::xread_activated() – change to process messages without 0
or 1 prefix, but without affecting subscriptions

zmq::xsub_t::xsend() – change to send rather than discard messages
without 0 or 1 prefix, but without affecting subscriptions

Update documentation
Merge pull request #494 from jgm-radez/master

allow XSUB/XPUB to send/recv messages unrelated to sub/unsub