Create a Conduit.encode! function

[christophermaier]

From @christophermaier on August 5, 2016 17:59

We use Conduit to encode all our MQTT messages to JSON for transmission, but just rely on them being structs in general; it's possible that a non-Conduit-enabled struct could be used, and then blow everything up.

Logic for checking this should be delegated to Conduit itself.

Copied from original issue: operable/cog#902