Scheduling Contextmanagers

``` python
with AIMDScheduler(queue) as sched:
  sched << job(4,5)
  sched << job2(5,6)
  sched.flush() # create a compound job of the above and push it to the queue
  sched << job3(6,7)
  # flush() on exit

# not specifying a queue is okay, << just won't work
# use sched as a callable
with TimeScheduler(timedelta(seconds=1000)) as sched:
  queue += sched(job(4,5))
```
