Fix build on OS X using all external libraries (except xerces)
Add installation command for easier packaging in future
Fix building on Linux using all external libraries (except xerces)
Added more directory to be installed so that the package is more complete.
overhauled the logging module.

- removed log message categories.
- added support for user-defined log formats.
- added facilities to save the configuration of a logger and automatically restore it when leaving the scope.
- by default, log messages are now prefixed with the date, time and thread id.
- warnings and errors (fatal and non-fatal) emitted during unit testing are now displayed.
fixed errors and warnings emitted during unit testing.
fixed typo.
renamings.
cosmetics.
overhauled the handling of non-physical lights and implemented directional lights.
added missing reference images.
added missing #include statement.
fixed thin lens camera's autofocus when the camera's transform has a non-unit scaling component.
print mouse coordinates (in pixels and NDC) when hovering over a render widget.
renderer::g_light_wavelengths is now a renderer::Spectrum instead of just being an array of floats.
set initial lights transformation to identity.
renamed a variable.
in appleseed, lights energy is defined in terms of radiance, not exitance.  renamed everything accordingly, except for the actual lights inputs in order to maintain compatibility.  a solution to this (input aliases?  extend normalizeprojectfile to upgrade scene files to the latest version?) will eventually need to be implemented.
implemented physically-based sun (as a directional light).
renamings and comment tweaks.
tweaked test scene.