move {ARTICLE,PAGE}_DIR -> {ARTICLE,PAGE}_PATHS

Instead of one path a list can be given. This is due to popular request.
Should help people not wanting to use Pelican for blogging.
Maintain backward compatibility though.
Merge branch 'get_files_multiple_paths' of https://github.com/smartass101/pelican into fix_1322

Conflicts:
	docs/settings.rst
	pelican/tests/test_generators.py