Fix TOTP token validation

* Removed support for number instances since they cannot represent all
  valid TOTP tokens.
* Fixed an issue where a valid TOTP token with a leading zero digit
  would be incorrectly considered as invalid.
Merge pull request #5 from seegno/bugfix/totp-token-numeric-validation

Fix TOTP token validation
Bump version
Add more specific errors

* Added `InvalidApiKeyAuthyError`
* Added `InvalidTokenUsedRecentlyAuthyError` inheriting from
  `InvalidTokenAuthyError`.
* Fixed `InvalidTokenAuthyError` inheriting from
  `InvalidRequestAuthyError`.