Doxygen: Added the cameras doc group.
Doxygen: Added the camera classes to the camera doc group.
Doxygen: Documented methods of the UEye_Camera class.
Doxygen: documented the Image class methods.
Makefile: Modified the 'doc' rule.

The documentation is now always regenerated by the 'make doc' command,
even if it was already generated.
Doxygen: added documentation for the utilities test.