public static DateTime? GetExpirationDate();
Retrieves an expiration date from the current license key. Returned
Nullable<DateTime> has a value if a current license key c ontains expiration date.
A license key becomes expired and ArmDot.Client.Api.GetLicenseState returns
LicenseKeyState.Expired if a current date is more than an expiraton date.
ArmDot.Client.Api.PutKey must have been already called.