examples: improve 'scores' example
calc: improve help screen.
calc: conditionally compile '--debug' option.

To enable it, Use
    ./configure CFLAGS="-DENABLE_DEBUG"
maint: update copyright year, add GPL
refactor: extract common functions into modules
build: fix minor errors
calc: new global case-sensitive option '-i' + tests
Merge branch 'master' into dummy_TravisCI_macOS