Adding MQTT library

[TopNiz]

I would like to contribute by a library to handle MQTT protocol. It is very useful for making easy networking projects like games or IoT projects. I have setup a complete library and an example project to explain how it is used.
In addition to all this, the implementation is mainly based on blocks besides some necessary Javascript blocks. It uses an Object Oriented Programming implementation of Snap! which can be a good demonstration of what is exposed on OOP in the manual.
Finally, this library shows also an effective way to make users program callbacks to custom events using C-Shaped blocks.
I tried a pull-request from my forked repository, but I'm denied access this git repo.

[TopNiz]

See #2561