Add calc_priority() and check_yield()
clamp priority value between PRI_MIN, PRI_MAX
Add recent_cpu and load_avg
Fix bug to compile
Enable 4.4BSD by -mlfqs. Recalculate priority per second.
small fix
Convert TAB to Space
Add design document for solution 1
small fix
debug
Merge remote-tracking branch 'origin' into jocelin
Merge branch 'cobra' of github.com:Cobra-Kao/pintos into jocelin

Conflicts:
	src/threads/thread.c
	src/threads/thread.h
debugging