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.
fixed crash (introduced in previous commit) when loading a scene after having opened the Render Settings dialog.
the watch_folder.py script can now forward additional, user-supplied arguments to appleseed.
tweaked comment.
the alpha mode of a texture instance may now be set to "detect": the renderer will try to guess which alpha mode to use based on the actual texture contents.
minor tweak in the output of appleseed.cli -h.
fixed parsing of valued command line arguments supporting multiple occurrences (such as the -p parameter in appleseed.cli).
Merge remote-tracking branch 'upstream/master' into ray_differentials
cosmetics.
fixed bug introduced in f75b987efb8301af5277476f9164227db9d00f61 ("simplified the code by making the const version of renderer::TransformSequence::earliest_transform() return an identity transform if the sequence is empty") and simplified the renderer::TransformSequence API.
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.