By Hongbiao Yang
 Changes to be committed:
	modified:   Makefile
	modified:   commands.c
	modified:   commands.h
	modified:   simfs.c
	deleted:    simfs
	changeLog:
	+ add command "sh" with 3-args implementation, redirect the output
	  into a file if a third argument is given; otherwise print the
	  output to terminal
	+ add a "\n" if command write is called explicitly
	+ modify a bug that resulting mess code at the end of the write
	  buffer