Added a signal mode to allow initiation of captures from another process.
Waits on SIGUSR1 between captures.

Fixed a possible issue with black frames in timelapse mode where
-tl 0 is used. Ensures the first frame has plenty of time to get its exposure
right.
Merge pull request #103 from JamesH65/signal_mode2

Added a signal mode to allow initiation of captures from another process...