First increment:

Add a basic set of event emitter to the client.

possible workers events are:

• worker register success
• worker unregister
• poll start
• poll error
• poll success
• handler error

possible taskclient events are:

• complete success
• complete errror
• handle failure success
• handle failure error
• handle bpmn failure success
• handle bpmn failure error
• extend lock time success
• extend lock time error