Add send/recv funcionallity

Hi @fagg ,

I have merged your `dev` branch with my `api/messaging-basics`. All tests are passing under `Windows + MATLAB` and `Ubuntu + Octave`.

Unfortunately I could not write a standalone test with socket types other than REQ/REP because MATLAB single thread nature... But I've added the folder `examples`, and all the examples can run successfully on the test environment.

As soon as possible I will rewrite `zmq_recv.c` to mimetize `zmq_send.c` conventions.

Best Regards.