Add a standalone irnetproxy.py script for the RedRat irNetBox III

The irNetBox only allows one network connection at a time.  This means
the 16 ir ports it has cannot be easily controlled by multiple clients
simultaneously.

This script allows many clients to connect to the device, and use
the ports concurrently.  **Note**, this only makes sense for the
Asynchronous IO Output Commands, other commands will be run
synchronously as before.

http://www.redrat.co.uk/products/IRNetBox_Comms-V3.X.pdf

Usage
-----

```
$ irnetproxy -r <irNetBox address>
```

OR

```
$ irnetproxy --irnetbox-addresss <irNetBox address> --irnetbox-port 10001\
      --listen-port 10001 --listen-address 127.0.0.1 -vv

> Listening for connections on 127.0.0.1:10001
```

Existing irNetBox network clients should then connect to the proxy
server, rather than the irNetBox.  Everything else should behave the
same.

Features
--------

 - Session multiplexing: irnetproxy dynamically maps Asyncronous Sequence IDs
 on the fly, meaning that multiple clients can use the same Sequence ID
 concurrently, and the irnet box will continue to work.  Both the command
 acknowledgement, and the subsequent Async complete message are always routed
 to the correct client.

 - On/Off management: irnetproxy will silently accept the on and off messages
 (5 and 6).  A standard ACK response is returned to the client, but the command
 is never sent to the device. This allows multiple scripts to try to turn off
 the device while other devices use it.  irnetproxy issues the ON command when
 it connects, and issues an OFF command when it is stopped.