Updated man pages
Added zsock_send/recv

Like zactor_send/recv, these methods operate on zmsg_t objects. We could
later expand zsock and zactor instances to accept zframes, zchunks and
other resolvable types (it might be nice to receive a full zhash from a
socket for example). To be seen...
And updated man pages again
Merge pull request #476 from hintjens/master

Added zsock_send/recv methods