Create a ScheduledTaskQueue to manage future scheduled tasks stored in memory

[bensheldon]

This is another take at #155.

The intention is to allow a Scheduler to store the times of future scheduled tasks and trigger a wake at those times.

My goal is to allow polling to be significantly dialed back (~1 minute), if not done away with entirely and allow the :async execution mode to be non-noisily set as the default in the Rails development environment. (#139 and #90)

This is given as an example, but with further research I believe that the ScheduledTaskQueue is not necessary because a ScheduledTask can be provided an explicit TimerSet that functions similarly to what I invented here.