generator.categories is now a list of (category, articles) instead of a dict. This
is to avoid using ordered dicts that have been introduces in python 2.7, so we stay
as much as possible compatible with older versions.
This fixes#62. Thanks to Bruno for the report.
-keep output directory if keep commandline switch is provided
-added KEEP_OUTPUT_DIRECTORY to settings (default is to delete output
directory before processing)
-updated documentation