Indexing service API and GUI improvements!

- New APIs: waitingTasks, completeTasks, task payload
- GUI for the above, and for task logs + status
TaskStorage: Return recently complete tasks in reverse chronological order
add better messaging and error handling
better messaging to console again
Configurability of recency threshold
Merge pull request #327 from metamx/index-service-api-gui

Indexing service API and GUI improvements!