Added a MQTT client utility class

It implements the automatic reconnection process, simplifying the
development of clients.
Added an implementation of MQTT-RPC (first implementation)

Inspired from the Javascript MQTT-RPC implementation at
https://github.com/wolfeidau/mqtt-rpc
(interaction not yet tested)

2 Topic per exporter:
  - pelix/remote-services/mqttrpc/<frameworkid>/request
  - pelix/remote-services/mqttrpc/<frameworkid>/reply

As other RPC implementations, the method name is "endpoint_name.method".