Add support for scheduling an event at specified DateTime.

[mihaimyh]

[jamesmh]

There are other considerations with a feature like this. A list a few out and see what you think:

• Should this feature even be part of the scheduler? What about using the Queue feature and having a Queue().ReleaseAt(DateTime) or something like this? (This would also require database persistence for queued items which is something that is on the todo list).
• If the scheduler is used to do this, what happens on app startup when the datetime to trigger a task configured to runat a specific time only is in the past? Do we still schedule it anyways?
• What happens after this task is completed successfully? Should we remove it from the list of configured tasks?
• What kinds of tests scenarios should be covered?

If you want to think about those, maybe look at how some of those things are done internally, and let me know what you think?