Add a tarball maker script

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
mktarball: minor enhancements

use a tarball/ directory
.gitignore it
verify pwd is right

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
v0.4

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>