refactor: prepare AccountStorage for more complex tokens

With this the AccountStorage module now fully supports some HOTP/TOTP parameters which are uncommon (but still part of HOTP/TOTP specifications).

 - Better types for offset, tokenLength. Make this consistent throughout
 - Finish support for offset, checksum parameters for HOTP tokens in AccountStorage
 - Finish support for hashing algorithm, epoch parameters for TOTP tokens in AccountStorage
 - Better API for creating oath::Algorithm instances
 - Code formatting (break up long lines)

Issues: #7