Expose TOTP window start

[miloush]

Implements #46

Currently there is no way to retrieve elapsed, remaining or absolute time of a window for TOTP. The RemainingSeconds() method returns an integer and is therefore not satisfactory to trigger actions on new time window (such as updating UI).

While Step is now available, the calculation of required values is non-trivial and requires knowledge of implementation details (such as the used epoch).

This PR adds WindowStart() method that returns absolute time of the current TOTP window. That way, users can get:

• DateTime of when current window started ... WindowStart()
• elapsed TimeSpan as WindowStart() - now
• remaining TimeSpan as now + TimeSpan.FromSeconds(Step) - WindowStart()