Begin adding control for multiple exposures

Photo kicks off exposures.
The main loop checks for how many are remaining and set time for next
exposure.
This is somewhat working but reporting excessively and erroneously in
the main loop the "Exposing now at:" message while counting down the
exposures. It is close enough that I though I should commit it.
Beging making non blocking control of shutter release

Makes multiple shutter releases, N and stops.
Has a few extraneous and incorrect messages about the motor going and
stopping.