Properly pass value of $SCREEN to as_user

Now passes the value of the variable rather than the literal '$SCREEN' to the as_user function when user requests the screen command.

(useful when this script is used to manage multiple instances for the same user in which case the screen attached would not necessarily be the one intended as the SCREEN variable was not used properly)
Merge pull request #163 from Bisa/patch-2

Properly pass value of $SCREEN to as_user
Merge remote-tracking branch 'upstream/master'