Add support to get confirmation of downlink message

[FokkeZB]

See https://www.npmjs.com/package/mqtt#mqttclientpublishtopic-message-options-callback

[FokkeZB]

@johanstokking is there a way we can get confirmation the downlink published is received by TTN?

[johanstokking]

We have two different stages of feedback in the making:

• Feedback events, see https://github.com/TheThingsNetwork/ttn/issues/159
• End-to-end confirmation, see https://github.com/TheThingsNetwork/ttn/issues/158

[FokkeZB]

Closed by #31