add challenge solutions
Release Lab 4
gradelib: Use bytes and bytearray types where appropriate

(patch 7c63faebf50a515f54d6a3fff63111f5034c77c0 from Austin)
finished ex.1-5
Part A Exercises Complete
user mode pgflt handler complete
lab4 part2 complete
All exercise complete for Lab4.
Merge branch 'lab4' of https://github.com/wky/jos into lab4

Conflicts:
	README
	kern/init.c
	lab3.html
	lab3.md