Previously, in order to specify a device with the timer, it had to be
specified in the timer's construction or had to be specified every time
GetElapsedTime was called. The first method was inconvienient in the
case where there are multiple code paths to define the device and the
latter method was inconvienient because you would have to pass around a
device id.

Both these techniques still exist, but we have also added a new form of
Reset that allows you to change the device the timer is used on.