Task timeouts and retries

Sometimes we want to retry a task on another worker if it appears to be taking a long time. One approach would be to specify a timeout when submitting the task

```
future = e.submit(func, *args, timeout=5)
```

Another would be for the function itself to raise a special exception

```
def func(*args):
...
raise RetryException()
```

The latter is somewhat attractive because it removes some administrative burden from the scheduler.

Pinging @minrk for thoughts or previous experiences.