Added Stop/Pause states and HDR option

- Included Stop and Pause states during time-lapse in process. The
Stop/Pause buttons must be pressed at the end of an interval, i.e. once
the last picture of the interval has been taken

- Included HDR functionality by including a middle step in the setup
which asks for how many pictures per interval, only 1 or 3 can be
selected. If HDR is used, a fixed 1 second pause is provided between
consecutive bracketed photos.