Merge pull request #1 from yumminhuang/borui

Modify thread, thread_create() and thread_set_priority()
Merge branch 'master' into yummin
Modify codes in src/thread

1. Add comments and DESIGNDOC
2. Rename member in thread struct
3. Reformat code
Merge branch 'yummin'
Answer questions in DESIGNDOC
Answer the remaining questions in DESIGNDOC
Merge branch 'yummin'
Merge remote-tracking branch 'origin/master' into borui

Conflicts:
	src/threads/DESIGNDOC
	src/threads/synch.c
	src/threads/thread.c