Modify files in src/threads

1. Add DESIGNDOC;
2. Re-implement timer_sleep() in devices/timer.c to avoid busy-waiting;
3. Modify threads/thread.c
Update branches/yummin
Modify thread.c and thread.h
Modify schedule() in thread.c
Passed tests for timer_sleep()
Modify synch.c and thread.c

Add some declarations for implementing thread priority
Implement thread priority scheduling
Fix the problem that cause 'priority-donate-multiple2' test failed.
Merging from trunk and add some comments.
Merging from trunk to branches/yummin
Merge branch 'yummin'
Merging branches/yummin to Master