Merge pull request #149 from pamod/mqtt_dev

Adding licence headers, adding functionality to rollback during an excep...
issue fixed in trying to deliver already bufferred message
Merge pull request #150 from indikasampath2000/performance

issue fixed in trying to deliver already bufferred message
refactored slot based modifications
Merge pull request #151 from sajinidesilva/refactor2

refactored slot based modifications
Refactoring code, adding ability to handle acks in MQTT
MQTT code refactoring