Add warning message if z.sh is executed instead of sourced.

Closes #74
Add Makefile for README
Use _Z_CMD in z -h
-c option restricts matches to subdirs of cwd.

Closes #83
Make -c without args work like regular z does
test directly, instead of testing exit variable
Allow options to be combined e.g -clt
update README to match man page
Remove extra . from man page.
Be explicit that -c matches should start with cwd, not just contain it.
chmod -x z.sh
Deal with spaces in $HOME