Adds callback as method of class

Fixes #2 

- define consumer / blocking callback with `$mb->consumeMessage(array(new MBC_UserAPICampaignActivity(), 'updateUserAPI'));`
- uses `MessageBroker::sendAck($payload);` for AcrBack (does this look right?) based on messagebroker-phplib enhancements
- removes __construct from MBC_UserAPICampaignActivity class, instance of MessageBroker is now created in body of script with `$mb = new MessageBroker($credentials, $config);`