added validation functions valid_hotp() and valid_totp(); added 100% test coverage; added option to generate zero-padded string tokens; changed names of OTP generation functions
added validation functions valid_hotp() and valid_totp(); added 100% test coverage; added option to generate zero-padded string tokens; changed names of OTP generation functions

tadeck/onetimepass,angelosarto/onetimepass,shantanoo/onetimepass