version bump 0.6.18: xor password

- Makefile structure aligns with js-xlsx
- introduction of password option (will change)
- support for 40-bit xor encryption
- alignment of common bits with js-xlsx
- SSF updated to 0.6.5
- more XLS properties
- jshint passes
- XLS and XLML custom properties tests pass
- XLS and XLML do not report empty ranges