Support ordering pages and articles when iterating in templates.
Order can be set to a metadata attribute or a sorting function.
Default to order by slug for articles and order by filename for pages.
Add test for PAGE_ORDER_BY
Merge remote-tracking branch 'remotes/malept/davidmarble-page-order-by' into davidmarble-page-order-by

Conflicts:
	docs/settings.rst
	pelican/generators.py
	pelican/tests/test_generators.py
Add missing methods
Address code review comments from PR getpelican/pelican#1348

The text about the sort-by-key function comes from:
https://docs.python.org/2/library/stdtypes.html#sequence-types-str-unicode-list-tuple-bytearray-buffer-xrange

Minor style cleanup as well.
Merge branch 'master' of https://github.com/getpelican/pelican into davidmarble-page-order-by
Addressed comments from @avaris in PR getpelican/pelican#1348
Fixes wrongly generated relative urls for pagination.

When `RELATIVE_URLS` is `True` and the `PAGINATION_PATTERNS` adds folder to the path, the relative urls to these pages does not respect these additional folders.

Example:
	Settings:
			RELATIVE_URLS = True
			PAGINATION_PATTERNS = (
					(1, '{base_name}/', '{base_name}/index.html'),
					(2, '{base_name}/page/{number}/', '{base_name}/page/{number}/index.html'),)

	Theme:
			<link href="{{ SITEURL }}/theme/css/style.css" rel="stylesheet">

If you are on page 2 then "{{ SITEURL }}/theme/css/style.css" expands to "./theme/css/style.css" instead of "./../../theme/css/style.css".

Fix:
Simply compute the relative url from the paginated url instead of the not paginated one.
Documents DOCUTILS_SETTINGS and initializes it. Fixes #1425
Merge pull request #1348 from vjousse/davidmarble-page-order-by

 Support ordering pages and articles when iterating in templates.
Merge pull request #1366 from Scheirle/paginated_relative_urls_fix2

Fixes wrongly generated relative urls for pagination.
Merge pull request #1426 from barrysteyn/DOCUTILS_SETTINGS

Documents DOCUTILS_SETTINGS and initializes it. Fixes #1425
Fix for broken test in #1348
Merge pull request #1470 from avaris/fix_1348

Fix for broken test in #1348
README: Added Coveralls badge, refactor Travis badge
Merge pull request #1374 from singingwolfboy/readme-badges

README: Added Coveralls badge, refactor Travis badge
Update faq.rst

since author feeds are enabled per default http://docs.getpelican.com/en/latest/settings.html#id9 as well, these need to be added here as well.
Merge pull request #1473 from ingwinlu/patch-1

Update faq.rst to include missing Author Feeds
EXTRA_PATH_METADATA path separator is OS-specific

Updating docs for EXTRA_PATH_METADATA to clarify that OS-specific path
separators must be used as keys, unlike some other Pelican variables.
Refs # 1133.
Spelling correction in docs: usefull -> useful.