Beginning of an MQTT client service
MQTT Connector Factory: Handle MQTT topic subscription filters

- Now using the Paho MQTT client module to check if a listener
subscription matches the topic
- The Connector PID is now a constant in pelix.services: mqtt.connector
Added an MQTT bridge for EventAdmin

All events from EventAdmin are published to all MQTT servers known by the MQTT connector.
The bridge, like EventAdmin, must be instantiated manually.
Merge branch 'dev' into ecf_compat