TOTP constructor
@param {secret} @type {String} @desc random base32-encoded key to generate OTP.
@param {interval} @type {int} @desc the time interval in seconds for OTP. This defaults to 30.
@param {digits} @type {int} @desc the length of the one-time password. This defaults to 6.
@return {TOTP}
Implementation
TOTP(String secret, [int interval = 30, int digits = 6]) : super(secret, digits) {
this.interval = interval;
}