Add Ruby scripts for font file obfuscation

Hi!

This commit adds two Ruby scripts that can be used to conveniently obfuscate font files according to either the Adobe od IDPF font obfuscation algorithm.

The scripts can be useful in keeping mangled font files in `epub30-test-0103` up to date (see commit 9a28cdc2fa314df698599a707dec6701cfab3e63).