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.