merge
Merge branch 'master' of github.com:JinSun2014/A_Tiny_Shell
Add execv command
add output
implement cd command
mMerge branch 'develop-YuchaoZhou'
add addToList and PrintList functions and tested successfully
implement bg jobs, but not test it
fix list pop bug
using enum parameter instead of char in changeStatus()
implement fg and test it a little
implement bg function, test it in primery version test and fix warnings
solve conflict
Fix two warnings: remove unused variable, modify a typo