Add simple MQTT support to subscribe to, publish messages.

This agent is a proxy to an MQTT broker.

I've set up a test instance on mqttcloud.com:

![screenshot from 2014-06-03 11 55 15](https://cloud.githubusercontent.com/assets/365751/3155628/99e0617e-eac6-11e3-913b-9d04c43436b5.png)

I've installed [owntracks](http://owntracks.org/) and configured it to point to the public cloud instance on my phone.

![rosewater](https://cloud.githubusercontent.com/assets/365751/3155644/1d22a2a4-eac7-11e3-9a44-6c8318ce361c.png)

I'm also running [TheThingSystem](https://github.com/TheThingSystem/steward) and publishing to the same MQTT broker - events like temperature/commands from my samsung airconditioner are being published.

![screenshot from 2014-06-03 12 02 53](https://cloud.githubusercontent.com/assets/365751/3155661/7971e61e-eac7-11e3-8b03-4be8dad57a3a.png)

![screenshot from 2014-06-03 12 03 10](https://cloud.githubusercontent.com/assets/365751/3155659/73992a04-eac7-11e3-9cae-e514c00951a9.png)


I haven't quite looked at ninjablocks and MQTT in detail yet, but that would add presence sensors, more temp/humidity sensors, window/door sensors, etc into the mix.


You can see the events it is sending around:
![screenshot from 2014-06-03 11 54 51](https://cloud.githubusercontent.com/assets/365751/3155668/9109dd0e-eac7-11e3-8088-d9443a679631.png)

Hopefully the documentation is understandable enough to get you started in a few steps:
![screenshot from 2014-06-03 11 53 45](https://cloud.githubusercontent.com/assets/365751/3155669/95f35b6a-eac7-11e3-9ab1-6e3852f0ae86.png)