cleanup: rename package and vendor dependencies for old go versions

Fixes #8 and begins to address deployability.

vendor/github.com/pebbe/zmq4/socketget_unix.go

@@ -0,0 +1,15 @@
+// +build !windows
+
+package zmq4
+
+/*
+#include <zmq.h>
+*/
+import "C"
+
+// ZMQ_FD: Retrieve file descriptor associated with the socket
+//
+// See: http://api.zeromq.org/4-1:zmq-getsockopt#toc9
+func (soc *Socket) GetFd() (int, error) {
+	return soc.getInt(C.ZMQ_FD)
+}