Initial support for zmq_msg_t

As mentioned in #78, current semantics of Recv are not sufficient for centain
scenarios, e.g. you want to receive a pointer over PAIR and then copy the data,
exactly as zmq_socket_monitor is supposed to be used. The issues is that every
call to Recv automatically closes the zmq_msg_t it uses, which is not desirable
here.

This tiny initial implementation decouples Recv from zmq_msg_close call, making
it possible to actually use zmq_socket_monitor and such.

Fixes #78.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>

Btw tests are intentionally left out, if this is refused :-) I will add them later if it's the other way around...