fire-cat: Improve shutter/gain/exposure control

Previously, fire-cat let you specify shutter and gain in the config arguments,
and would use these values if specified (non-zero). There was also an
--exposure command line switch which used an ad-hoc software method to compute
the shutter parameter requried to give a certain exposure.

The ladybug and bumblebee cameras allow for auto exposure via the libdc1394
drivers so I'm wondering why this wasn't used originally. If there are other
libdc1394 devices currently using fire-cat that do not support auto-exposure,
then this patch should be re-considered.

This pull-requst does the following

-  Replaces the overloaded `dc1394::set_exposure`` method with
   ``dc1394::set_relative_shutter_gain`` and ``dc1394::set_absolute_shutter_gain``
   and adds ``dc1394::set_exposure``.

-  Uses `relative_shutter`, `absolute_shutter`, `exposure` in that order of
   preference.

-  If exposure is set to 0, auto exposure is used, otherwise the value is used 
   as a manual exposure setting. Note setting "exposure" causes the camera to
   automatically adjust the shutter and gain to meet the exposure target.

-  Removed the --exposure argument and placed it in the config list alongside 
   `shutter` and `gain`.

Additionally, this request includes the command line fixes of #5.