Add a groupId listener

[levibostian]

From the discussion in this issue, adding a global listener for a specific groupId would be beneficial to be able to listen to a group of tasks and when they will all be complete.

Tasks to help you get started:

• Create a listener interface with the listeners
  • Create callback functions for: runningTask(task: PendingTask), fun allTasksComplete(), fun taskDoneRunning(taskId: Long, successful: Boolean, numberTasksLeftInGroup: Int)
• Add function in WendyConfig to add a listener to a group by id
  • When listener added, trigger a callback with the current status of the groupId like done in other add listener functions.
• Add function to remove listener
• Call callbacks in the task runner.