Add the ability to sort categories.

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.

pelican/settings.py

@@ -18,6 +18,7 @@ _DEFAULT_CONFIG = {'PATH': None,
                    'FALLBACK_ON_FS_DATE': True,
                    'CSS_FILE': 'main.css',
                    'REVERSE_ARCHIVE_ORDER': False,
+                   'REVERSE_CATEGORY_ORDER': False,
                    'KEEP_OUTPUT_DIRECTORY': False,
                    'CLEAN_URLS': False, # use /blah/ instead /blah.html in urls
                    'RELATIVE_URLS': True,