Respect period parameter in otpauth URI.

_From @thomasdarimont on October 5, 2015 8:26_

According to the [Key Uri Format documentation](https://github.com/google/google-authenticator/wiki/Key-Uri-Format#period) the current google authenticator implementation
doesn't use the `period` parameter in the otpauth URI and defaults to 30 seconds.

This makes it difficult to use the google authenticator app as an alternative soft token generator mechanism in scenarios where one wants to use one OTP token policy in combination with hardware token generators which often come with a hard-wired default period.

A fallback is to use other apps like FreeOTP which respect the `period` parameter.

_Copied from original issue: google/google-authenticator#521_