Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
- correct test period from 100ms to 1000ms, to match description
- moved description to separate README.md
- changed timer message type from 44 to 42, for nerdiness
- minor code enhancements
