Merge pull request #205 from kayousterhout/logging

Added logging of scheduler delays to UI

This commit adds two metrics to the UI:

1) The time to get task results, if they're fetched remotely

2) The scheduler delay.  When the scheduler starts getting overwhelmed (because it can't keep up with the rate at which tasks are being submitted), the result is that tasks get delayed on the tail-end: the message from the worker saying that the task has completed ends up in a long queue and takes a while to be processed by the scheduler.  This commit records that delay in the UI so that users can tell when the scheduler is becoming the bottleneck.