Introduce all_articles where articles is shadowed.
update the docs regarding all_articles
Make METADATA_PROCESSORS and EXTENSIONS public.
Allow to load custom readers with new dict setting: READERS, associating
a file extension to its new reader.
revert as we can now directly edit pelican.readers.EXTENSIONS
Merge branch 'master' of ssh://github.com/Starch/pelican

Conflicts:
	pelican/readers.py