Problem: need client API for send/recv

Solution: build this to match protocol messages

    int
    mlm_client_stream_send (mlm_client_t *self, char *subject,
                            zmsg_t **content_p)

Note: needs latest CZMQ master.

Fixes #19
Problem: mshell isn't using new client API

Solution: fix mshell to use the msg send/recv API
Problem: perftest isn't using new send/recv API

Solution: fix perftest to use this.
Merge pull request #26 from hintjens/master

Problem: need client API for send/recv