Push out sysfs updates over MQTT

[mhemeryck]

First attempt:

• poll changes with the sysfs interface
• push out any changes over MQTT

[mhemeryck]

Next steps:

• [x] return values from functions: proper error handling
• [x] test: test the function APIs?
• [x] MQTT receiver: have 2 threads: one for receiving, another for sending
• [x] map file event to a named topic, message?
• [x] count the duration a file is in given state?
• [x] cross-compile for arm

[mhemeryck]

Instead of having everything hard-linked together with synchronous calls, I'm going to have a look at how to use channels to pass events along.

[mhemeryck]

Done in #4