mod_mqtt: integrated mqtt support.
This optionally starts a MQTT listener, using emqtt.
The MQTT router is shared between sites, each adding their own topic tree.
Browser pages have their own simple local routing mechanism, and can dispatch observe or publish commands to the server's router.
Communication between the server and pages is done using the normal event mechanism, where payloads are encoded using UBF(A).

Issue #642
core: fix include of mqtt header file.
core: rename mqtt auth module to something emqtt expects.
mqtt: better access check integration with the mqtt listener.
mqtt: missing from last commit.
core: start with better handling of page process re-starts.
core: also stop emqtt before stopping mnesia.
mod_mqtt: add simple record/object mapping to ubf.js
mod_mqtt: remove console.log, update todo-list
Merge branch 'mw-mqtt'