Implement a basic scheduler

[dave]

Marvin needs a basic scheduler, something on which to build other enhancements. The basic scheduler should enable task creation (at any point during execution), do preemptive round-robin scheduling, and gracefully handle task ends (ie. not crash when a task function returns).

This is the "meta-task" that will be crossed off when all the small steps are made. See other tickets for small steps in question.

Note to self: look at eCos for a lovely implementation of a scheduler lock to enable safe manipulation of online task information.

[dave]

Done as of 54918218125b.