New picture message methods for zsock

Goal is to reduce work needed to send/receive rich commands to actors. This is a last piece in simplifying the zactor API and making it unnecessary to build a custom method API.

Old v2 style:

    zbeacon_publish (node1, (byte *) "NODE/1", 6);

New v3 style:

    zsock_send (node1, "sbi", "PUBLISH", "NODE/1", 6, 250);

See zsock_send for picture specs.