raspicam: option to enable stats pass on captures

If a stats pass is done on stills captures, then digital gain
and AWB are recomputed based on the actual capture frame statistics.
Otherwise it uses the values from the preview frame immediately before
the capture.
Positives: can reduce the time required between start and capture as
  only AGC has to vaguely converge.
Negatives: will increase processing time for captures and cause a shift
  in white balance and gain for the capture (ie don't spend a long
  time getting preview looking perfect if this is set as things will be
  recomputed on capture).

The dynamic lens shading algorithm currently wants a stats pass too,
although it will cope without.

Default is no stats pass, with -st option enabling it.
Raspicam: Add option to force the sensor mode.

Useful in some odd situations to force field of view
or binning modes.
Select with -md or --mode on the command line.
Merge pull request #204 from 6by9/PR-20140919

Raspicam: options for stats pass and forcing the sensor mode