Added MQTT subscriptions. Can subscribe to multiple topics.
This isn't working quite as I would like, need to look into
it more to work out why. Can't seem to get `connection.get` to work
when I pass in a topic variable ie. `connection.get(topic)`. So
currently I'm setting the topic to `#` which is the search all
topics. Then filtering using `event_topic_name`. Not ideal as we
lose the nice MQTT topic features such as hierarchy and wildcards.