Updates docs for Python 3

In Python 3, the simpleHTTPServer has moved into the http module, so the command `python -m simpleHTTPServer no longer works. This minor change adds instructions for those of us using Python 3.
add <ul> to make valid html
Enables custom locale while feed generation.

Fixes wrong language of month names in feed links.

Related to #1198
Updates the custom_locale test feed files to match the correct month names
minor spelling fixes
fix the parameters for a logging.warning call
Make PAGE_EXCLUDES work with subdirs. Fixes #1500.

The old code was naively comparing the strings in PAGE_EXCLUDES to the
subdirectory names produced by os.walk(). (Same with ARTICLE_EXCLUDES.)
This had two surprising effects:

Setting PAGE_EXCLUDES=['foo'] would exclude all directories named foo,
regardless of whether they were in the top-level content directory or
nested deep within a directory whose contents should not be excluded.

Setting PAGE_EXCLUDES=['subdir/foo'] would never exclude any directories.

In other words, there is no way to exclude a subdirectory without risking
the accidental exclusion of other directories with the same name elsewhere
in the file system.

This change fixes the problem, so 'subdir/foo' and 'foo' will be distinct
and both work as expected. If anyone out there is depending on the old
behavior, they will have to update their settings. I don't expect it to
affect most users yet, since Pelican doesn't yet make nested directory
structures very useful. When it does, this fix will become important to
more people.
rework summary attribute

*   write summary property in modern style
*   introduce get_summary which is memoizable, based on localsiteurl
*   introduce get_siteurl
Merge pull request #1501 from foresto/subdir-excludes

Make PAGE_EXCLUDES and ARTICLE_EXCLUDES work with subdirs. Fixes #1500.
Merge pull request #1466 from cgar/spelling

Minor spelling fixes
add paragraph mentioning markdown extensions to docs
Document Markdown inclusion in Quickstart install
Merge pull request #1358 from ingwinlu/md_exten_add

Improve docs regarding Python-Markdown extensions
Merge pull request #1361 from ingwinlu/rewritten_summary_2

Re-work summary attribute to be memoizable
Warn that signal receivers are weakly-referenced

For the signal get_generators in particular, it may seem natural to use
a lambda or inner function as the signal receiver, but this does not
work as the receiver is collected before it can be called.
Merge piwik.html into analytics.html

Addresses #1512.
Merge pull request #1518 from iKevinY/merge-analytics

Merge piwik.html into analytics.html
Merge pull request #1457 from svenkreiss/master

Add <ul> to authors.html template so HTML is valid
Minor documentation improvements
Mention the INDEX_SAVE_AS setting in FAQ and direct templates

This setting can be particularly useful when making a static homepage.